A pulse signal is produced in a pulse generating section, and repetitively transmitted from an antenna at constant time intervals. A reception signal, received through an antenna and including a reflective wave from an object, is amplified and gain-adjusted. Thereafter, a distance detecting section detects a reception pulse of the reflected wave, and calculates the distance to the object. The variable attenuator attenuates the reception signal by a gain adjusted in accordance with a gain control signal. The gain of the variable attenuator is controlled so as to be maximum immediately after the pulse signal is transmitted, and to be reduced with elapse of time. Based on a gain adjustment timing signal, a gain adjustment timing when the attenuation amount is changed is made different for each transmission of the pulse signal.