1. A PLC comprising: an I/O module to which an external device is connected; a CPU module for executing an execution program controlling the I/O module; a connector for electrically connecting the I/O module and the CPU module to each other; and a displace device including a display screen, wherein the CPU module includes: a first storage unit which previously stores an intrinsic control program in which an operation program indicative of operation processing corresponding to the I/O module and an input/output information item indicative of the operation processing are packaged; a second storage unit which previously stores a function block representing the operation processing by a single symbol; a third storage unit which previously stores an equivalent input/output information item equivalently representing the input/output information item by a graphical language; an ID number acquisition unit which reads out ID number of the I/O module electrically connected to the connector; a program creation unit which calls the control program corresponding to the read ID number and automatically creates the execution program from the called control program; and a main storage unit which stores the execution program created by the program creation unit, and wherein the display device includes a display unit which causes the execution program stored in the main storage unit to be displayed by using the function block and the equivalent input/output information item by referring to the second storage unit and the third storage unit.