PURPOSE: To perform accurate time control even when a clock having a shorter period than the execution time of instructions is used by comparing the counted value of a clock counter with an elapsed time value from a main storage.

CONSTITUTION: A clock 10 as a trigger for updating an elapsed time value is counted by the counter 3. A CPU1 executes one instruction and then checks on the contents of the counter 3 before the execution of a next instruction. When the contents of the counter 3 are zero, the next instruction is executed. When the contents of the counter 3 are ≥1, the CPU1 reads the elapsed time value 13 out of the main storage 2 and subtracts it from the counter value. When the result is positive, the result is regarded as a new elapsed time value. When the result of the calculation is zero or minus, an initial value is read out of the main storage 2 and its sum value is re-stored in the main storage 2 as the new elapsed time value.