PURPOSE: To correctly and surely execute simulation for every task and every tag to constitute a program through simple operation in a controller to control various kinds of plants.

CONSTITUTION: The simulation designation (simulation cancelation designation) of a tag or task unit is executed from a monitoring device 3, and in this controller 1, the extraction of an input/output data area 7 being used in the task or tag designated from the monitoring device 3 and the switching (simulation designation) of this input/output data area 7 from the usual actual input/output data area 8 to a corresponding simulation data area 9 are executed. Further, a simulation task is generated automatically.