PURPOSE: To eliminate distortion present in the abutment surfaces of a wheel and a contact by joining the contact to an angle detecting arm via a regulation means which is allowed a degree of freedom in a plane in which it abuts to the side face of the wheel, and allowing the means some degree of freedom only when it abuts to the wheel.

CONSTITUTION: A wheel is clamped and a roller 13 is made to abut to the abutment part M of the side face of the wheel. As the fixed state of the middle point of the wheel is monitored, the eccentricity of the wheel is corrected. A regulation means 12 is allowed some degree of freedom and the roller 13 to be disposed in a sensor arm 11 is held in a movable state via the means 12, and then the wheel is rotated by one turn. The roller 13 is driven by the wheel, rolling its roller main body 131 in the direction perpendicular to the direction N of its roller axis. In a state in which distortion that causes upward movement of the roller 13 may occur, for example, the roller 13 escapes upward and shifts to its U-position. Therefore, when the roller 13 is fixed, unremovable distortion is absored within the amount of movement of the roller 13, so preventing distortion of the abutment surfaces. Thereafter, the wheel is stopped and is clamped on both sides.