The invention provides an inner groove gauge which comprises a locating guide sleeve, an ejector rod, a measuring rod, a spring, a sphere, a dial gauge provided with a telescopic pushing rod and a dial gauge base. The locating guide sleeve is provided with a peripheral face matched with a workpiece hole to be measured; the locating guide sleeve is provided with a first cavity and a second cavity; the first cavity and the second cavity are arranged in an orthogonal mode; the ejector rod is located in the first cavity and is matched with the first cavity; the measuring rod is located in the second cavity and matched with the second cavity and extends out of the second cavity; the two ends of the spring abut against the ejector rod and the dial gauge base respectively; the dial gauge base is fixedly connected with the locating guide sleeve; the dial gauge extends into the first cavity through the dial gauge base; the telescopic pushing rod abuts against the ejector rod; the end, in contact with the sphere, of the measuring rod is a conic hook face or a plane, wherein the included angle between the conic hook face and the central axis of the measuring rod is 45 degrees or the included angle between the plane and the central axis of the measuring rod is 45 degrees; the sphere is located in the first cavity and is in contact with the ejector rod and the measuring rod. The inner groove gauge is simple and compact in structure and accurate in measurement.