In a measuring jig for measuring an inner diameter of a valve hole portion formed coaxially with a guide hole, the measuring jig includes: an insertion shaft inserted into the guide hole; and a measuring instrument having a probe that is disposed around an axis of the insertion shaft and that is brought into contact with an inner wall of the valve hole portion. The inner diameter of the valve hole portion is measured by way of the probe by turning the measuring instrument about the axis, and the measuring instrument is disposed on the axis.