PROBLEM TO BE SOLVED: To measure the plane state of the upper jaw (a member to be detected) provided in the mouthpiece of an attachment member by an inertia measuring instrument.SOLUTION: The apparatus for measuring the plane state of the upper jaw is constituted so that an inertia measuring instrument (13) and an attachment member (11) are provided at the central position (1A) of a plane plate (1), and the flat state of the upper jaw (member to be detected) provided in the mouthpiece (12) in the attachment member (11) is measured by the inertia measuring instrument (13).