To measure the period of a pulse signal, without invalidating data obtained in a detection period, even if noise is applied in that period.

Every time an edge detection signal is outputted by an edge detecting element 1, time data measured by a detecting timer 3 is stored in an evacuation area 6 accumulatively, and the counted value of a Hi- or Lo-level counter 7 or 8 is increased in accordance with the level of a pulse signal judged by a signal level judging unit 5 at every sampling time. If the above-mentioned level judged first by the signal level judging unit 5 after the edge detection signal is outputted is normal, the duty ratio of the pulse signal is judged on the basis of the counted values of the Hi- and Lo-level counters 7 and 8. And, when the duty judging unit 9 judges it normal, the time data being stored in the evacuation area 6 is written, stored, and held in a holding area 10 as pulse interval times, and along with it content stored in the evacuation area 6 is cleared.