A scanning optical apparatus has a source (1) of light for producing a light beam as a light flux (5) in a given direction, a polariscope (2) for polarizing the light flux (5) of the light beam in another given direction, a member or medium which is scanned by the light flux (5) of the light beam while the light flux (5) of the light beam is polarized by the polariscope (2), and a plastic lens (3) placed between the polariscope (2) and the scanned member or medium (4) for forming an image spot on the scanned member or medium (4). The plastic lens (3) has two kinds of surfaces, one of which is a polarization surface or optical surface through which the light flux (5) of the light beam passes, and the other of which is a non-polarization surface or non-optical surface through which the light flux (5) of the light beam does not pass. All of the surfaces of the plastic lens (3) have dampproofing characteristics.