In an apparatus using an intensity-modulated light for detection of spatial information based upon light intensity of light reflected from a target space, a timing synchronization circuit is provided to synchronize a phase of the intensity-modulated light from a light-emitting element with a timing of operating a light-receiving element receiving the intensity-modulated light. The light-receiving element is caused to operate for enabling the detection of intensity of the received light for each of a plurality of phase regions within one cycle of the intensity-modulated light. The timing synchronization circuit functions to compare a cyclic variation determining the operation of the light-receiving element with a cyclic variation associated with an output from a light-emitting element driving circuit in order to keep a constant phase difference between these two cyclic variations.