An optical scanning device having a deflector with an oscillating element wherein a relation 0.85<(dY/d.theta.)/(dY.sub.0/d.theta..sub.0)<1.15 is satisfied throughout an effective scan region, where .theta..sub.0 is a deflection angle of the oscillating element as a light beam scans the center position Y.sub.0 of an effective scan region on a scan surface with respect to a main-scan direction, .theta. is the deflection angle of the oscillating element as the light beam scans a scan position Y in the effective scan region with respect to the main-scan direction, dY.sub.0 is an amount of change at the center position Y.sub.0 on the scan surface with reference to an amount of change d.theta..sub.0 of the oscillating element at the deflection angle .theta..sub.0, and dY is an amount of change at the scan position Y on the scan surface with reference to an amount of change d.theta. of the oscillating element at the deflection angle .theta..