PURPOSE: To improve the measuring accuracy thereby to effect the digital display in any unit and to facilitate the reading-out operation by measuring with the use of a fourth counter the time period for the instant when the counting operation of a third counter is started to the instant when the counted value becomes coincident with that of a second counter.

CONSTITUTION: The period of a first impulse train is measured by a first counter CNT 1. The time difference between any impulse in a second impulse train and the impulse in a first impulse train having arrived just before is measured by a second counter CNT 2. A third counter CNT 3 starts the counting operation of the output signals of a feed circuit in accordance with the arrival of the impulse of a first impulse train which arrives at first after the arrival of the impulse of the second impulse train. The time period from the start of the counting operation of the third counter CNT 3 to the time when the counted value becomes coincident with the counted value of the second counter CNT 2 so that the counted value of a counter CNT 5 is displayed in response to the coincidence signals.