Provided is an opening measurement instrument which exhibits excellent instrument stability during measurement, and excellent measurement accuracy and reproducibility thereof. This opening measurement instrument is provided with: an engagement part which measures an opening amount between an upper jaw and a lower jaw, and which engages with first reference teeth in one of the jaws a measurement surface which, while the engagement part is in a state of being engaged with the first reference teeth, is brought into contact with second reference teeth in the other jaw and a first scale which is provided to the measurement surface, and which indicates the distance from the engagement part. The opening measurement instrument is preferably further provided with a second scale which is provided to the measurement surface, and which indicates the position of a direction orthogonal to the direction in which the first scale is provided. The measurement surface is preferably configured from a curved surface in which the measurement surface between the engagement part and a prescribed point of the measurement surface bulges with respect to a flat surface connecting the engagement part and the prescribed point of the measurement surface. Furthermore, the curved surface is preferably provided with a section having a curvature which increases as the distance from the engagement part increases.
