A programmable integrated circuit of the present invention can change the input system of serial/parallel input-parallel output circuit for program data from serial to parallel or vice versa in response to a control signal from control signal input. Therefore, a program can be written at a relatively low speed through the parallel output of serial input, and a program can be written at a relatively high speed by inputting data in parallel and outputting the input data in parallel. Moreover, the bit width of the aforementioned serial/parallel input-parallel output circuit can be changed in response to a control signal from control signal input, whereby bit width for data input, shift and the like can be optimized according to the quantity of programs to be written with the result of improved freedom of users and the reduced time required for writing a large quantity of programs.