PURPOSE: To improve receiving sensitivity by an apparatus wherein a laser beam is divided into a carrier beam and a local oscillation beam, and a beat signal is obtained from the local oscillation beam and the reflected beam from the object to the carrying beam having been subject to pulse modulation and frequency conversion.

CONSTITUTION: The output beam from a laser 11 is divided into a carrier beam Lc and a local oscillation beam Ll by a beam distributor 12. The carrying beam Lc is applied to an acoustic beam reflector 13 to be modulated with a high frequency signal from a high frequency generator 14. An output diffraction beam Ld from the deflector 13 is applied to a beam modulator 17 via a slit 15 and a lens 16 to be pulse-modulated and then sent out in the running direction of a vehicle from a transmission optical system 19. A reflected beam from the object is received by a receiving optical system 20 and fed to a receiving element 22 after being superimposed with the local oscillation beam Ll in a beam mixer 21. A beat signal eB obtained by the element 22 is fed through an intermediate frequency amplifier 23 to a detector 24 where it undergoes envelop detection. A video signal ed thus obtained is fed to an information processing circuit 25 together with a trigger signal et in synchronism with a modulation signal em of a pulse modulator 18. The circuit 25 receives ground speed information from a speed sensor 26 and issues an alarm (a) if a possibility of collision be detected.