The value of fixed cycle time is adjusted to an optimum value in a short period of time. In a second operational mode (MON) of a PLC, a series of a plurality of system processes are repeatedly performed with a change to a user program in a peripheral service process being allowed. A change to fixed cycle time to be compared with actual cycle time is prohibited in a first operational mode (RUN) but is allowed in the second operational mode (MON) and a non-operational mode (PRG).