The over-field type optical scanner includes a first imaging optical system disposed between a light source and a rotating polygon mirror for forming, on one surface of the rotating polygon mirror, a linear image with a width larger than a width along a main scanning direction of the one surface of the rotating polygon mirror; and a second imaging optical system composed of a curved mirror for focusing, on a scan surface, a light beam having been reflected by the rotating polygon mirror. The first imaging optical system includes a first conversion optical system for converting the light beam emitted from the light source into a divergent light beam with respect to the main scanning direction and into a convergent light beam with respect to the sub scanning direction; and a second conversion optical system having a refracting power in the main scanning direction.