A first plastic lens and a second plastic lens constituting each imaging optical system are arranged on opposite sides of an optical deflector, so that main scanning directions of optical beams scanned by the single optical deflector become substantially parallel to each other, and the first plastic lens and the second plastic lens of at least one of the plurality of (four) imaging optical systems are formed such that secondary components at scanning positions on the (four) surfaces to be scanned are arranged in a same direction, and are molded by a same mold cavity.