A spatial information detecting device using intensity-modulated light comprising: at least one photoelectric converter for receiving a light provided from a space into which a light intensity-modulated at a predetermined emission frequency (ω1) is being irradiated, and outputting a receiver signal with a signal level corresponding to an intensity of received light, an intensity Y1 of the receiver signal being represented by the following equation: Y1 = b1 + a1*cos (ω1*t + ψ), wherein "ψ" designates a phase difference between the phase of the irradiated intensity-modulated light and the phase of said receiver signal, "a1" designates a constant corresponding to an amplitude of said receiver signal, and "b1" designates a constant corresponding to dark current or outside light; a local oscillator circuit for outputting a local oscillator signal having a local oscillator frequency (ω2) different from the emission frequency (ω1), an intensity Y2 of the local oscillator signal being represented by the following equation: Y2 = b2 + a2*cos (co2*t) wherein "a2" designates a constant corresponding to an amplitude of said local oscillator signal, and "b2" designates a constant corresponding to a direct bias; a sensitivity controller for mixing said local oscillator signal with said receiver signal to obtain a beat signal represented by Y1 * Y2 and having an envelope component corresponding to a difference between the emission frequency (ω1) and the local oscillator frequency (ω2); characterized by comprising an integrator for determining integrals of said beat signal, wherein the integration is performed a number of times within one period of the beat signal; and an analyzer for determining the phase difference "ψ" from the integrals of said beat signal to convert said phase difference "ψ" into spatial information. at least one photoelectric converter for receiving a light provided from a space into which a light intensity-modulated at a predetermined emission frequency (ω1) is being irradiated, and outputting a receiver signal with a signal level corresponding to an intensity of received light, an intensity Y1 of the receiver signal being represented by the following equation: Y1 = b1 + a1*cos (ω1*t + ψ), wherein "ψ" designates a phase difference between the phase of the irradiated intensity-modulated light and the phase of said receiver signal, "a1" designates a constant corresponding to an amplitude of said receiver signal, and "b1" designates a constant corresponding to dark current or outside light; a local oscillator circuit for outputting a local oscillator signal having a local oscillator frequency (ω2) different from the emission frequency (ω1), an intensity Y2 of the local oscillator signal being represented by the following equation: Y2 = b2 + a2*cos (co2*t) wherein "a2" designates a constant corresponding to an amplitude of said local oscillator signal, and "b2" designates a constant corresponding to a direct bias; a sensitivity controller for mixing said local oscillator signal with said receiver signal to obtain a beat signal represented by Y1 * Y2 and having an envelope component corresponding to a difference between the emission frequency (ω1) and the local oscillator frequency (ω2); Y2 = b2 + a2*cos (co2*t) wherein "a2" designates a constant corresponding to an amplitude of said local oscillator signal, and "b2" designates a constant corresponding to a direct bias; a sensitivity controller for mixing said local oscillator signal with said receiver signal to obtain a beat signal represented by Y1 * Y2 and having an envelope component corresponding to a difference between the emission frequency (ω1) and the local oscillator frequency (ω2); an integrator for determining integrals of said beat signal, wherein the integration is performed a number of times within one period of the beat signal; and an analyzer for determining the phase difference "ψ" from the integrals of said beat signal to convert said phase difference "ψ" into spatial information.