A scanning optical device includes at least one light source unit for emitting a light beam, a first deflector for deflecting the emitted light beam in an auxiliary scanning direction, a condensing optical system for generating an intermediate image of the light beam deflected by the first deflector, a collecting optical system for condensing a light beam diverged from the generated intermediate image, a second deflector for deflecting the condensed light beam in a main-scanning direction, and a scanning optical system for scanning the surface to be scanned with the light beam deflected by the second deflector. The condensing optical system has an f-θ characteristic, the collecting optical system has an f-sin θ characteristic, and the scanning optical system has an f-sin θ characteristic.