PURPOSE: To improve the noise characteristic and stability by arranging two band-pass filters set to a proper value respectively as band-pass filters and providing a switch switching the band-pass filters and a switch controller controlling the switching.

CONSTITUTION: A controller 7 gives a control signal to a switch 3 so as to select a band-pass filter 4a having a wider band of a band-pass filter 4. The pass band WF1 of the band-pass filter 4a contributes the noise characteristic of an asynchronous signal to be inputted but the C/N0 characteristic to a degree detecting a level of a signal with respect to noise has only to be obtained. The signal level in the pass band WF1 is monitored by the controller 7 and when a signal exists already in the pass band WF1, the frequency is swept to increase the signal level when the signal is at the outside of the pass band WF1 and to increase the signal level when the signal is within the pass band WF1. Thus, the frequency is swept at a prescribed step, then the signal is detected by the edge in the pass band WF1.