A gas detection that includes a projector including a light source that emits laser light having a wavelength at which an absorption rate by a detection target gas is high and a spatial light modulator that modulates the laser light emitted from the light source, a projection control unit that controls projection light to be projected toward a retroreflector by causing the light source to emit the laser light and setting a pattern of a modulation part of the spatial light modulator, a light receiver that receives reflected light of the projection light reflected by the retroreflector and measures an intensity of the received reflected light; and a leakage determination unit that acquires the intensity of the reflected light from the light receiver and determines leakage of the detection target gas in a detection space with the retroreflector based on the intensity of the reflected light.