PURPOSE: To economically diagnose a real time counter without stopping a computer system by measuring the counting period of the real time counter to be diagnosed by using an auxiliary counter independent of the real time counter and reading out the first and final values of the counting period from the real time counter to diagnose the real time counter.

CONSTITUTION: The auxiliary counter 2 smaller than the real time counter being a diagnostic object is provided. The value A1 of the real time counter 1 being a diagnostic object is read out by a start instruction signal 1 for starting the increment operation of the counter 2 and the value A2 of the counter 1 is read out again by a signal 2 obtained when the counter 2 detects the time completing the previously set increment amount to diagnose the normality of the counter 1 from the counted values A1, A2 of the counter 1 during the measuring period of the counter 2. Consequently, the operation of the real time counter 1 can economically be diagnosed without stopping the computer system.