A spatial information detection device is provided, which is capable of, even under the condition that an environmental light having intensity fluctuations exists in a target space, projecting a light intensity-modulated with a predetermined modulation signal from a light emitting source into the target space, receiving light from the target space with a photodetector, and detecting spatial information of the target space from a change between the light projected from the light emitting source and the light received by the photodetector. This device has a smoothing unit configured to integrate, over a predetermined integration period, a fluctuation component in a prescribed phase zone of the modulation signal with respect to an electric charge amount generated by receiving the environmental light in the electric charge amount generated at the photodetector by receiving the light from the target space, thereby smoothing the fluctuation component.