A sub-scanning interval adjusting apparatus for a multi-beam scanning unit adjusts an interval between at least two laser lines that are formed on a photoreceptor drum without causing starting points of image formation to vary. In the multi-beam scanning unit simultaneously emitting at least two laser beams by using at least two laser sources, the sub-scanning interval adjusting apparatus includes a transparent member varying in thickness depending on its height, in a direction where the laser beams pass through, the transparent member having an inclined lower side with respect to a scanning direction of the laser beams, and a movable member having an inclined side corresponding to the inclined lower side of the transparent member, thereby adjusting an interval between the laser beams by moving the movable member in the scanning direction and thus varying the height of the transparent member with respect to the laser sources.