A high frequency signal, which is produced in a oscillator (2), is modulated by a frequency from a modulating signal generated by a modulation signal generator (1) and radiated from a sending antenna (5). A reflected signal from a target object is received by a receiving antenna (6), and frequency-converted by a local oscillating signal from a directional coupler (4) in the frequency convertor. Then, distance and speed information is detected by a signal processing unit (8). A modulator (3), which is connected to the oscillator (2), modulates the oscillator output in order to detect a failure. A detection unit, in which a filter unit (10), a rectification circuit (11), and a voltage comparison circuit (12) are included, detects modulated elements originated in the above described modulator. The signal processing unit makes a failure determination based on the modulated elements which have already been detected.