A checking tool for measuring a distance between adjacent sinking grooves in an inner hole of a mechanical part, including: a gage block, end cover, sliding piece, pin, base, pin seat, support, measuring rod, spring, barrel, and dial indicator. An inner end face of the base is provided with the sliding piece, at least three concentric circular arc grooves are on the end face, paired eccentric arc grooves are on the sliding piece, the pin connected to the pin seat runs through the base and sliding piece, which is axially limited by the end cover, and the seat rotates relative to the base to realize radial expansion displacement of the sliding piece. A measuring head is formed by the dial indicator, measuring rod, spring and barrel. By positioning the edges of adjacent sinking grooves in an inner hole, measurement for a distance between the grooves is converted into depth measurement.