PURPOSE:To execute easily a capacity test of various tools by interposing a jig corresponding to the shape of the tool on surface table of a necessary size, and connecting the main shaft of a brake disk of a measuring part and an output shaft of the tool. CONSTITUTION:A grinding wheel and holding metallic fittings, etc. of a linear type air grinder B are removed, and a jig engaged to a head part of a main shaft 3 of a measuring part is installed to a grinder output shaft and fitted to the head part. A neck part of the grinder is loaded on a V-block 52 on a fitting base 51 of an installing device 50, and the main shaft 3 and the shaft core are made to coincide and fixed. A motor part is driven by feeding air to the grinder B, and a revolving speed in a no-load state is measured by a tachometer 46. Subsequently, a damping force is applied to a brake disk 5 from a hydraulic pump 22 by depressing a brake operating lever 23, and a pressure in case when the grinder B has been stopped is measured by a pressure gauge 36, by which a capacity test of the grinder is executed.