Besides a time constant circuit 5 that imposes a frequency limit on a wireless signal detected by a power detecting circuit 4 at a preset time constant Ta, there is provided a time constant circuit 8 that imposes a frequency limit on the wireless signal at a time constant Tb greater than the time constant Ta. When the signal level of an intermittent operation signal supplied from a signal input terminal 1 is H level or when a threshold processing circuit 6 decides that the level of the wireless signal is higher than a threshold Th, a logic unit 3 supplies a control signal that instructs starting to the power detecting circuit 4, time constant circuits 5 and 8 and threshold processing circuits 6 and 9.