A tooth flank machining device includes a helical teeth grinding wheel, a position adjusting unit] capable of moving a relative position of a rotational axis of the helical teeth grinding wheel to a rotational axis of a work gear, and a controlling unit having a relative position controller activating the position adjusting unit to adjust the relative position of the rotational axis of the helical teeth grinding wheel to the rotational axis of the work gear so that the helical teeth grinding wheel engages the work gear with a grinding wheel tooth flank of the helical teeth grinding wheel abutting on only one of work tooth flanks forming a tooth of the work gear, a grinding wheel rotating unit controller activating a grinding wheel rotating unit, and a torque controlling unit controller activating a rotational torque controlling unit to adjust rotational torque within a predetermined range.