Non-telecentric illuminating light obtained by controlling the shape of an opening of an illumination aperture is directed onto a photomask, and a characteristic such that an image of a pattern of the photomask formed by the non-telecentric illumination moves in the direction perpendicular to an optical axis when an image-forming plane is moved in the direction of the optical axis, to perform focus monitoring. This eliminates the need for a special photomask, so that inexpensive and highly precise focus monitoring method, focus monitoring apparatus, and a method of manufacturing a semiconductor device can be obtained.