To position a pinion cutter for a workpiece without causing circular arc interpolation error and to machine a gear in the workpiece efficiently and precisely using a machining center.

Each control shaft of the machining center is controlled by a numerical controller, and a pinion cutter T mounted on a main spindle moves to a plurality of machining positions p1, p2, p3, etc., set above the number of teeth of the gear sequentially by a pitch angle Δα obtained by dividing a shaft axis c2 along the peripheral direction of a circle G centered a shaft axis c1 of the gear to be machined in the workpiece W and is positioned. After the pinion cutter T is rotated and positioned by a pitch angle Δβ divided by corresponding to each machining position around the shaft axis every time it is positioned at each machining position, cutting feed is given in the direction of Z-axis and a tooth face of the gear is formed in the workpiece W by a cutting edge Tc, and the pinion cutter T goes round around the center c1 of the workpiece to complete machining of the gear.