1. A method for reading 2-bit lower and upper data bits programmed in memory cells, comprising: sequentially applying a word line voltage Vrd 3 and a word line voltage Vrd 1 to a selected word line to read out the lower data bit; applying a word line voltage Vrd 2 to the selected word line to read the upper data bit; and combining the read lower and upper data bits to output the programmed data bits.