To provide an optical scanner that has a multi-beam light source and a plastic-molded scanning lens, and suppresses a variation in light intensity on a scanning target surface.

A scanning optical system of a K station includes a resin-made scanning lens 2105a, and three folding mirrors (2106a, 2107a, and 2108a). A magnitude relationship between a reflectance ratio to light fluxes toward a scan start position and a reflectance ratio to light fluxes toward a scan end position in a photoreceptor drum 2030a is set so that the folding mirror 2106a has the inverted relationship to that of the remaining folding mirrors. A difference in a reflectance ratio between the maximum value and the minimum value in a relationship between a deflection angle and a reflection ratio of a polygon mirror 2104A is set so that the folding mirror 2106a has the largest difference of the three folding mirrors.