1. For an electrically alterable non-volatile multi-level memory device including a plurality of non-volatile multi-level memory cells, each of the multi-level memory cells including a floating gate FET having a channel with electrically alterable voltage threshold value, the plurality of non-volatile multi-level memory cells being disposed in a matrix of rows and columns, channels of multi-level memory cells of a first group of the plurality of non-volatile multi-level memory cells being coupled in parallel between a first bit line and a reference potential, channels of multi-level memory cells of a second group of the plurality of non-volatile multi-level memory cells being coupled in parallel between a second bit line and the reference potential, electrons being capable of being injected into the floating gate by a phenomenon of hot electron injection from the channel of each of the plurality of non-volatile multi-level memory cell, electric currents flowing through the channels of the multi-level memory cells of the first group and electric currents flowing through the channels of the multi-level memory cells of the second group being substantially flowing in a same direction, a method of operating the electrically alterable non-volatile multi-level memory device, comprising: settling a parameter of at least one non-volatile multi-level memory cell of the plurality of non-volatile multi-level memory cells to one state selected from a plurality of states including at least a first state, a second state, a third state and a fourth state in response to information to be stored in the one non-volatile multi-level memory cell, verifying whether the parameter of the one non-volatile multi-level memory cell has being settled to the one state selected from the plurality of states by comparing the parameter of the one non-volatile multi-level memory cell with a plurality of verifying reference parameters including at least a first verifying reference parameter, a second verifying reference parameter, a third verifying reference parameter and a fourth verifying reference parameter, and of repeating the operation for settling the parameter and the operation for verifying until it is verified by the operation for verifying that the parameter of the one non-volatile multi-level memory cell has being settled to the one state, reading status of the one non-volatile multi-level memory cell by comparing the parameter with a plurality of reading reference parameters including at least a first reading reference parameter, a second reading reference parameter and a third reading reference parameter, wherein a conductivity value of the one non-volatile multi-level memory cell is decreased in order of the first state, the second state, the third state and the fourth state, wherein the first reading reference parameter is allocated between the first state and the second state, the second reading reference parameter is allocated between the second state and the third state, and the third reading reference parameter is allocated between the third state and the fourth state, wherein the first reading reference parameter, the second reading reference parameter and the third reading reference parameter are parameters for a normal read operation in which the information stored in the one non-volatile multi-level memory cell can be read out by output data of a plurality of bits, wherein the normal read operation is carried out by parallel-comparing the parameter with the plurality of reading reference parameters by using a plurality of sense circuits including at least a first sense circuit, a second sense circuit and a third sense circuit, first input terminals of the first sense circuit, the second sense circuit and the third sense circuit are commonly supplied with the parameter from the one non-volatile multi-level memory cell, a second input terminal of the first sense circuit is supplied with the first reading reference parameter, a second input terminal of the second sense circuit is supplied with the second reading reference parameter and a second input terminal of the third sense circuit is supplied with the third reading reference parameter, and wherein the first verifying reference parameter is allocated below the first reading reference parameter, the second verifying reference parameter is allocated between the first reading reference parameter and the second reading reference parameter, the third verifying reference parameter is allocated between the second reading reference parameter and the third reading reference parameter and the fourth verifying reference parameter is allocated above the third reading reference parameter.