A programmable controller includes two CPU units each having a detachably attached special-function module for carrying out a calculation process, each of the two CPU units recognizing conditions of the other CPU unit, one of them becoming an active unit and the other becoming a standby unit such that when the active unit fails and goes down the standby unit is switched to become active and resumes operations. The special-function module attached to the active CPU unit becomes an active module and carries out control processes with the active CPU unit and the other special-function module attached to the standby CPU unit becomes a standby module.