1. A programmable logic controller comprising: (a) a program memory for storing a program expressed in the form of a combination of a first instruction code for indicating the kind of process operation and a second instruction code containing modified data to specify the operating conditions of the process operation of the kind expressed by said first instruction code; (b) a plurality of external numerical data setters for manually entering numerical data relating to said operating conditions; (c) an input device for writing any desired program into said program memory and for entering a specific code indicating external data has been entered in at least one of said data setters so that an external data reference command is stored in a predetermined area of said modified data; (d) a central processing unit (CPU) for sequentially reading out such instructions stored in said program memory and for sending out control signals in conformity with said instructions, the CPU having connecting means for connecting said plural external numerical data setters to said CPU; (e) reference command code detection means for checking whether external data reference command is present in said predetermined area of modified data of said instruction read-out prior to the execution of the instruction read-out from said program memory; (f) numerical data read-out means to read-out the numerical data set at a data setter when such an external data reference command is detected; and (g) replacement control means replacing the part or the whole of the modified data of that instruction containing said external data reference command code in the modified data with data readout from said external numerical data setter.