A scanning optical device includes a scanning lens, a deflector, and a frame configured to support the scanning lens and the deflector. The deflector includes a substrate, a motor fixed to the substrate, and a polygonal mirror rotary driven by the motor. The scanning lens is arranged such that a longitudinal direction thereof is oriented in a main scanning direction of the deflector. The frame includes a supporting part configured to support the scanning lens. The supporting part is located such that the substrate and at least a portion of the supporting part overlap when viewed in a rotation axis direction of the motor.