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

A dustproof coat 95 having electrical conductivity and translucency is formed to cover the front surface of dustproof glass 90 and is retained by projecting parts 100a which are portions of a casing 100. The casing 100 and a main body 150 of an exposure unit 14 mounted with the scanning optical device 15 are connected by a conductor 140. Since the main body 150 is grounded, the charges electrified to the dustproof coat 95 are removed through the casing 100 and the conductor 140.