To actualize a system switching control device applied to a CPU duplex system for the control device which can be structured at low cost and change the type of a CPU module.

This system is equipped with an abnormality monitor circuit 24 which holds a control transition indication display value and normal/abnormal state display values and control/standby state display values of its system and the other systems and outputs a CPU normal signal A1 of its system CPU by monitoring the state of a device of its system, a transmitting and receiving circuit 22 which sends and receives CPU normal signals A1 and B1, status signals A2 and B2, and control mode signals A3 and B3 indicating control transition indication display values of its system and the other system, a duplex system switching circuit 25 which performs system switching processing according to the CPU normal signals A1 and B1, a control mode signal B3 of the other system, and the held state display values and outputs a control mode signal A3 to a register group to hold it, and a PCI bus interface 23 which is connected to a compact PCI bus 3 and can access the abnormality monitor circuit 24.