The present invention relates to an optical scanning device in which a light beam is incident twice on a deflective reflecting facet and its object is to reduce curvature in scanning line trail or make the curvature substantially zero, or correct scanning line displacement due to the curvature in scanning line trail. Two stationary plane mirrors (13, 14) are disposed to face a deflective reflecting facet (11) which can be rotated about its rotational axis (12) such that a light beam (a1) being incident on and reflected from the deflective reflecting facet (11) is reflected by the two stationary plane mirrors (13, 14) sequentially. The reflected light beam (a3) is incident on and reflected by the deflective reflecting facet (11) again. Assuming that a plane being parallel to the rotational axis (12) and including the light beam (a0) which is first incident on the deflective reflecting facet is an incident plane, the central ray of an emergent light beam (a4) when the emergent light beam (a4) after the second reflection by the deflective reflecting facet (11) is on the incident plane and a straight line as the central ray of a light beam being projected on the incident plane when the deflective reflecting facet (11) is revolved by the maximum rotational angle are set substantially parallel to each other.