Abstract: In an optical distance-measuring apparatus wherein the light modulated with the constant frequency f.sub.o is emitted through an oscillator OSC, an amplifier SA and a light emitting diode LED and is returned by a reflecting prism DP located on the point the distance to which is being measured, and the distance is measured by the phase difference between the emitted light and returned light, a phase control circuit PCC is provided between the oscillator OSC and the amplifier SA for sequentially inverting the phase of the emitted light A. The real phase difference .psi.A is obtained by averaging the phase difference .psi.A of the normal light A and the phase difference .psi.A.sub.2 of the inverted light A. The averaging operation cancels the error component .delta..sub.1 due to the electrical or optical signal leakage in the light transmitting system.