To make detection by a measuring head without interrupting machining by a tool and shorten the machining cycle time in a machine tool provided with a radius measurement sizing device.

The tool 16 is fed toward the cylinder part Wa of a rotating work W to machine the cylinder part Wa, and the measuring head 30 of the radius measurement sizing device S is allowed to abut on the cylinder part from one side of the radial direction to measure the diameter. The measuring head invariably abuts on the cylinder part being machined to detect the diameter, the measured data of the detected diameter are stored in a memory means at the prescribed measurement time intervals, and the measured data are compared with the predetermined diameter data to control the feeding speed of the tool. The average values of the measured data stored in the memory means are calculated at the calculation time intervals larger than the measurement time intervals, and the average values may be compared with the predetermined diameter data to control the feeding speed of the tool.