PURPOSE: To simplify a positioning operation and to improve the accuracy of positioning, by conducting the positioning in the direction of a component force directly without positioning a constant position of a reference position.

CONSTITUTION: The amplitude of direct-current input signals Dx and Dy of a two component combining circuit 3 is varied in accordance with the direction of a component force vector, and thereby an alternating-current output signal A having a prescribed phase difference from a reference phase signal is formed. Meanwhile, a resolver 11 receiving the reference phase signal as an input signal is connected to the main axis of a work piece W to be tested, and it delivers to a phase comparator 12 a signal having the same period as the reference phase signal and the same phase with the rotational phase of the work piece W. In the comparator 12 said signal is compared with the signal A of the circuit 3, and a phase difference between these signals is detected and supplied to a motor driving control circuit 13. The circuit 13 makes a motor 6 rotate so that said phase difference may turn to be zero, and thereby the directions of first and second component force vectors are makes it unnecessary to conduct an operation for positioning once the reference position of the work piece W at a definite position, simplifies a testing operation, and enables the improvement in the accuracy of positioning.