A gear grinding unit, capable of grinding a tooth flank of a gear to have fine surface roughness without requiring any special shaping work such as dressing, includes a threaded grinding part which is rotatable around a rotation axis (S1) and engages with a tooth of a gear to be ground. Through a rotational motion of the threaded grinding part, a tooth flank of the gear is ground. The threaded grinding part includes a plurality of flexible abrasive sheets each including a virtual circle having a diameter that equals a root diameter (DA) of the threaded grinding part and a bulge part bulging radially outward from the virtual circle. With the bulge parts of the abrasive sheets mutually shifted around the rotation axis (S1), the abrasive sheets are overlapped along the rotation axis (S1) to form the threaded grinding part.