Device for optically measuring distance

In a device 100 for measuring distance, a signal generating circuit 20 generates a driving signal. The driving signal is serially modulated in phase into at least three states. The signal generating circuit 20 further generates reference signals having frequencies the same as the driving signal. A light source 30 is driven by the driving signal to emit detection light which is time-divisionally modulated in phase in accordance with the driving signal. An optical detector 40 receives the detection light which has been emitted from the light source 30 and scattered and reflected by the target. The optical detector 40 generates a detection signal which is time-divisionally modulated in phase in accordance with the received detection light. Homodyne processing circuits 50 and 60 superimpose the detection signal and the reference signals to generate homodyne signals (interference signals). Temporal mean values of intensities of the homodyne signals are obtained. A gain controlling circuit 80 feed-back controls the gains in the circuits 50 and 60. A phase detecting circuit 70 measures the temporal mean values of intensities of the homodyne signals, and calculates a difference between a phase of the detection light emitted from the light source and a phase of the detection light received at the optical detector.