To provide an aperture measuring instrument which is excellent in the stability and measurement accuracy of the instrument during measurement and its reproducibility. The opening measuring instrument measures an opening amount between the upper jaw and the lower jaw, and a locking portion locked to the first reference tooth in one jaw, and the other in a state where the locking portion is locked to the first reference tooth The measurement surface with which the 2nd reference tooth in a jaw contacts, and the 1st scale which is provided in a measurement surface and shows the distance from a locking part is provided. The aperture measuring instrument is preferably further provided with a second scale that is provided on the measurement surface and indicates a position in a direction orthogonal to the direction in which the first scale is provided. The measurement surface is preferably formed by a curved surface in which a measurement surface between the locking portion and a predetermined portion of the measurement surface swells with respect to a plane connecting the locking portion and the predetermined portion of the measurement surface. The curvature of the curved surface preferably has a portion that increases as the distance from the locking portion increases.測定時の器具の安定性及び測定精度とその再現性に優れる開口測定器を提供すること。 開口測定器は、上顎と下顎との開口量を測定し、一方の顎における第１基準歯に係止される係止部と、係止部が第１基準歯に係止された状態で他方の顎における第２基準歯が当接する測定面と、測定面に設けられ、係止部からの距離を示す第１目盛と、を備える。開口測定器は、好ましくは、測定面に設けられ、第１目盛が設けられる方向に直交する方向に関する位置を示す第２目盛を更に備える。測定面は、好ましくは、係止部と測定面の所定の箇所との間の測定面が、係止部と測定面の所定の箇所とを結ぶ平面に対して膨らんだ曲面により構成される。曲面の曲率は、好ましくは、係止部から離れるに従って大きくなる部分を有する。
