To prevent the degradation in image quality due to dust and dirt sticking to dustproof glass on an optical path of a laser beam without lowering the productivity substantially.

This scanning optical device is arranged with a dust removing roller 80 which has a long hole 82 penetrating in a diametral direction and is rotatable so as to come into contact with dustproof glass 75. In exposure, the long hole 82 and a long hole 72 of a casing 70 closed by the dustproof glass 75 are put into the state of being communicated with each other by rotation of the dust removing roller 80. While not in exposure, the long hole 82 and the long hole 72 are put into the state of not being communicated with each other.