PURPOSE: To surely decide slippage occurring between a friction roller and a driven wheel by specifying circumferential speed of circumference line where a wheel and at least one friction roller mutually rotate and then comparing them with each other.

CONSTITUTION: Based on circumferential length measurement of a wheel, known position in axial direction where a measured circumferential length is located, and known history of actual profile, at least entire size of a wheel for at least one measurement is found, other circumferential line length in the contact range between a tread and a friction roller is calculated based on a measured circumferential line and actual profile history. Thus, based on the known position of a cutting tools, the length of circumferential line in the range of cutting tool is calculated. Similarly, its size is helpful for size control of a known measurement wheel, and at that tome, a rotation speed of a measured wheel, or a circumferential speed of a measurement wheel, is decided, and converted into a circumferential speed of circumferential line in the range of a bite. The circumferential speed is compared to that corresponding to the friction roller, and then slippage is decided.