Based on a left tooth surface (28) having a maximum accumulated pitch error ALmax (when clockwise direction is a positive direction) and a right tooth surface (29) having a minimum accumulated pitch error ARmin (when clockwise direction is a positive direction), which are conceived to most likely have insufficiently ground portions, a correction value δθ which cancels these accumulated pitch errors for suppressing the accumulated pitch error after grinding is obtained. A reference tooth groove angle θ0 is corrected by the correction value δθ, thereby obtaining the meshing angle θ of the work gear. All teeth detection and signal processing in a short period of time is realized by using a displacement sensor and a high-speed meshing-dedicated circuit board.