A plastic optical element for guiding a scanned light beam in a main scanning direction and a sub-scanning direction, which is prepared by a molding method using a die. The element includes: a main body including at least two optically functional surfaces, which are transfer surfaces formed by contacted with transfer surfaces of the die and through which the light beam passes; and at least two ribs located on surfaces of the main body other than the optically functional surfaces and extending in parallel in the main scanning direction. A recessed portion is present on a surface of at least one of the ribs, and the recessed portion has a side transfer surface and a bottom surface, which is a non-transfer surface, and the distance between the optically functional surfaces is less than the length of the main body in the sub-scanning direction.