An inner diameter measuring device for measuring a shape of an inner surface of a cylindrical member, comprising an image pickup unit (2) disposed on a base end side of a frame unit (10) and for picking up an image of a forward end side, a cone mirror unit (30) provided on a forward end side of the frame unit and having a cone mirror (29) with a conical reflection surface at a forward end, and a laser beam emitting unit (14) for emitting a laser beam (17) toward a forward end of the cone mirror, wherein the cone mirror unit can be replaced by another cone mirror unit having a cone mirror with a different vertical angle to suit measuring condition.