PURPOSE: To realize the measurement of the CPU processing time, by turning on the bit under measurement after the measurement valid bit is turned on to carry out an instruction by an instruction given from a CPU and starting a counter to stop this counter at reception of the next input/output instruction.

CONSTITUTION: A processing time measuring part 1-1 is added to a control part 1 of a communication system which connects plural terminals to a center. A control register group 2 includes a measured valid bit 2-1 and an under- measurement bit 2-2. The part 2-1 turns on the bit 2-1 to display the measurement of the processing time and carries out an input/output instruction given from a CPU by a processing time measuring instruction of the CPU. Then the part 2-1 turns on the bit 2-2 to display the under-measuring state of the processing time. A timer counter 3 is started and then stopped at reception of the next input/output instruction. Then the bit 2-2 is turned off for the measurement of the CPU processing time.