PURPOSE: To execute monitor without adding a transmission line for monitor between a monitor antenna and an SSR receiver, by juxtaposing the monitor antenna with an SSR antenna in proximity thereto and by transmitting a monitor signal to the receiver.

CONSTITUTION: In a place 30 of installation of an antenna, a monitor antenna 1 is juxtaposed with an SSR antenna 10 in proximity thereto. A transmission signal sent from the SSR antenna 10 is received by the monitor antenna 1. A reception signal 1A is delayed by a delay line 2 and mixed, as a delay signal 2A distinguished from a signal of an SSR main body, with a difference pattern channel signal 21A of an SSR an reception signal by a directional coupler 3. A mixture signal is passed through a receiving element 15 and sent to a monitor signal detecting element 4. The monitor signal detecting element 4 detects only a monitor signal 4A according to a gate signal 5A. The monitor signal 4A is sent to an alarm generating circuit 6 and an alarm is issued therefrom on the basis of the presence or absence of the monitor signal 4A.