To enable a tracing function to add time information to received data and compare the time information with time when an abnormality is generated in a distributed control system where a large number of packets flow in a transmission line in a short time because communications are performed between individual controllers.

A communication packet fetched from the transmission line is temporarily stored in a buffer and transferred to a memory by every fixed interval. The existence of the packet is checked in the buffer by every timer time by using a timer signal of the shortest time to fill the buffer and, when the packet exists, a CPU is interrupted, and the communication packet is transferred to the memory. When no packet exists, the CPU is not interrupted. Addition of time required for trace information, processing of the trace information, data compression, etc., are enabled by generating free time in the CPU by using the buffer and a timer.