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

In this scanning optical device, a shutter 80 which can close or open a long hole 72 formed at a casing 70 and a fan 87 which can supply clean air through an air supply port 90 to an outlet portion of the long hole 72 are arranged. In exposure, the long hole 72 is opened by rotating the shutter 80 and the clean air is ejected from the long hole 72 toward printing paper 2 by a fan 87. While not in exposure, the long hole 72 is held closed by the shutter 80.