A spatial information detecting device using an intensity-modulated light is provided. This device comprises a photoelectric converter for receiving a light from a space into which a light intensity-modulated at a predetermined emission frequency is being irradiated, and generating an electrical output corresponding to an intensity of received light; a local oscillator circuit for outputting a local oscillator signal having a local oscillator frequency different from the emission frequency; a sensitivity controller for mixing the local oscillator signal with the electrical output to frequency convert the electrical output into a beat signal having a lower frequency than the emission frequency; an integrator for performing integration of said beat signal at a predetermined timing; and an analyzer for detecting information concerning the space according to an output of the integrator. According to this device, it is possible to accurately detect the spatial information without using a switching element having high-speed response at the light receiving side.