A multi-beam scanning unit includes a light source having a plurality of light emitting portions, each light emitting portion emitting a laser beam, and a beam deflector deflecting each of the laser beams emitted from the light emitting portions in a main scanning direction of a photosensitive medium. The light emitting portions are arranged in a line on a light exit surface of the light source, and an angle A between a section on a light exit surface of the light source corresponding to a sub-scanning direction that is a direction in which the photosensitive medium moves and a section connecting the light emitting portions satisfies the following inequalities: 0°