PURPOSE: To generate the gear in the internal peripheral surface of the work with oblong section with a high accuracy by a method wherein the work is moved along the oblong trace by utilizing the planetary rotating motion of planetary gears while a tool having an axial line parallel to the axial lines of the planetary gears is rotated.

CONSTITUTION: When a spindle 8 and a driving gear 17 are rotated, the planetary gears 27 is revolved through a rotation regulating gear 21 and a crank shaft 20 while the planetary gear 27' is revolved through the rotation regulating gear 21, a free rotation gear 15, the rotation regulating gear 21' and the crank shaft 20' respectively. According to these motions, each pins 35, 35' on respective planetary gears 27, 27' describe the trace 35a of the long-circle, therefore, a slide-contact base plate 33 and the work W, fixed to the base plate 33, are moved along the oblong trace without accompanying rotations. The internal surface 39 of the work W is created into the oblong sectional configuration by the rotation of a cutter edge 9 on the spindle 8.