PURPOSE: To attain miniaturization of a detector by constituting the detector for loopback command signal of a counter circuit to attain the digitized and integrated circuit.

CONSTITUTION: The 1st and 2nd counter circuits 202, 203 detect the consecution of a single frequency signal from an inputted signal. When a signal repeated by a time Tf1 (Tf1 is a decided time of the counter circuit 202) is given from a reception input terminal 115 via a slicer 201, the counter circuit 202 is counted up after a prescribed time. When a signal repeated by a time Tf2 (Tf2 is a decided time for the counter circuit 203), the counter circuit 203 is counted up after a prescribed time, the output of the counter circuit 202 sets a flip-flop 111 for loopback control and the output of the counter circuit 203 resets it by receiving continuously a signal of a single frequency f1 (period Tf1) as a loopback-on command signal and a signal of a single frequency f2 (period Tf2) as a loopback-off command signal.