A relative position detection method detecting a position of a light-emitting member relative to a lens array or a position of the lens array relative to the light-emitting member, in which the light-emitting member includes light-emitting elements arranged in a first direction and the lens array includes lenses arranged in the first direction and condenses light from the light-emitting elements to image positions of the light-emitting elements, optical axes of the lenses being orthogonal to the first direction. The method includes: causing the light-emitting elements to emit light; detecting, at a position displaced from the image positions along an optical axis direction, optical intensity distribution of light emitted from the light-emitting elements and transmitted through the lens array; and detecting a position of the light-emitting member relative to the lens array or a position of the lens array relative to the light-emitting member with use of the optical intensity distribution.