PURPOSE: To reduce a phase error, by averaging the phase difference of a detection signal received without being passed through a delay circuit to the original detection signal and the phase difference of a detection signal received through a 1/2 wavelength delay circuit to the original detection signal.

CONSTITUTION: The original detection signal (a radio wave or sonic wave) from a signal generating part 1 is projected to an object 5 to be measured through a wave guide 2. The detection signal reflected from, transmitted through or scattered by the object 5 is received by a receiving part 7 through a wave guide 6. A receiving signal is inputted to a phase measuring part 8 and the phase difference between said signal and the original detection signal is measured. An arithmetic control part 9 operates a change-over part 3 and projects the original detection signal to the object 5 through a 1/2 wavelength delay wire 4. The phase difference at this time is also measured by the measuring part 8 and subjected to arithmetic mean along with the phase difference not passed through the delay wire 4. Whereupon, the phase error due to a leakage signal 20 is negated and measuring accuracy is enhanced. This apparatus is utilized in an ultrasonic range finder.