Provided is a threaded-hole inspection device having high inspection precision. An incomplete thread ridge portion of a gauge portion 5 of a thread gauge 1 is removed, a phase-positioning portion 8 for positioning a tip-side start point 7 of a complete thread ridge at a prescribed phase when the thread gauge 1 is mounted on a rotating-shaft portion 2 is provided to a fastening portion 6 of the thread gauge 1 that is to be fastened to the rotating-shaft portion 2, and a phase-positioning engaging portion 9 that is to engage with the phase-positioning portion 8 is provided to the rotating-shaft portion 2. A depth of a threaded hole 4 to be inspected is measured on the basis of a number of rotations and thread pitch of a rotating-shaft-feeding mechanism 18 that, via threaded engagement, feeds the rotating-shaft portion 2 in a threaded-hole-approaching direction, which is a direction of approach toward the threaded hole 4, the rotating-shaft portion 2 being caused to rotate by a rotary drive portion 3, and on the basis of a distance over which an outward-moving portion 23 moves outward from the threaded hole, and the depth of the threaded hole 4 is calculated.