A container mouth inspection device which can produce an ideal optical image that reliably provides information on the inner contour of the container mouth, and which enables accurate and speedy measurement of the inside diameter or the like of the container mouth. The container mouth inspection device includes a light source (2) for emitting diffused light to the bottom (99b) of the container (99) having a mouth (99a), and an optical system (1) in which a lens (10) and a diaphragm (11) are arranged along an optical axis (5) extending through the center of the mouth (99a) of the container (99) such that the diaphragm (11) is positioned behind the lens (10). The diaphragm (11) is offset backward from a back focus of the lens (10) along the optical axis (5) by a predetermined distance d so as to form an optical image of the mouth (99a) in a position behind the diaphragm (11).