A control unit includes: a memory; a computation control part; and an input and output unit for connecting the computation control part to a device of an (FA) system. A system program includes a data refresh program for executing a data refresh of data of the device and the control data of the memory via the input and output unit. If the computation control part receives a change of a control program, the computation control part stops executing the control program. The computation control part executes the change of the control program while executing a part of the system program including the data refresh, and resumes executing the control program.

A control unit includes: a memory; a computation control part; and an input and output unit for connecting the computation control part to a device of an (FA) system. A system program includes a data refresh program for executing a data refresh of data of the device and the control data of the memory via the input and output unit. If the computation control part receives a change of a control program, the computation control part stops executing the control program. The computation control part executes the change of the control program while executing a part of the system program including the data refresh, and resumes executing the control program.