A programmable controller is provided with a controller body including a microprocessor having a ROM for storing user programs connected with a socket thereto. A programmer which is connected by a connector to a data bus or the controller body has a means to switch to a program mode to write a user program in a RAM by operating keys, and a test run mode to execute the user program written in RAM on the controller body, and a means to copy the user program written in RAM of the programmer to said ROM. The user program stored in ROM which is inserted in said socket is executed when the programmer is isolated from the controller body. The system program of the controller body includes a basic control function program for executing a single loop control, a cascade control function program for executing the cascade control and a selective control function program for the selective control, so that the control operation function is decided by selecting one of said control operation functions programs by a single instruction of the user programs.