By setting elements within the range that predetermined conditions are satisfied, for example, so that a size of a rotating polygon mirror is minimized, the rotating polygon mirror is made compact while the eclipse of light beams in the main scanning direction is prevented. The cost reduction of an apparatus is thus realized. The compact rotating polygon mirror reduces the consumption energy and the amount of heat generated in its drive system. Deteriorations in various optical characteristics including an increase in spot diameter of the light beam by temperature variation, uneven scanning pitch, and sub-scanning direction variation in beam pitch are suppressed.