A multibeam exposure device carrying out exposure processing by irradiating, onto an exposure surface of a photosensitive material, an exposure beam obtained by modulating a light beam, by a spatial light modulator and in accordance with an image to be exposed and formed, the multibeam exposure device having: an opening plate having an opening disposed on the exposure surface and blocking light which is other than an object of measurement of light quantity data at the spatial light modulator, the opening allowing passage of the exposure beam which corresponds to a pixel which is an object of measurement of light quantity data at the spatial light modulator; a feeding operation mechanism moving the opening plate such that the opening is moved in a direction intersecting a scanning direction at a time of scan-exposure; and a light-receiving element measuring a light quantity of the exposure beam which passes through the opening.