To allow a belt winding apparatus of a safety belt device to be inexpensive and lightweight by making it as a reduction gear having a simple structure.

In a harmonic speed reduction gear composed of a eccentric body 12 on a driving side which are engaged within a opening 13 complemented by an outer gear member 14 and an inner gear ring 15 fixed on a housing side and disposed around the outer gear member 14, and rolling in the opposite direction of rotation of the eccentric body 12 along a tooth surface when the eccentric body 12 is rotating, the outer gear member 14 is allowed to have two teeth 16, 17 disposed on the opposite side in a generally diameter direction. Therefore, in each state at the rotation of the eccentric body 12, a first tooth 16 is pressed in a tooth roots part 18 of the inner gear ring, and a second tooth 17 is detached from the inner gear ring and rolling in the opposite direction of the rotation of the eccentric body 12. By further rotation of the eccentric body 12, the second tooth 17 is pressed in the tooth roots part 18 to continue the action of detaching the first tooth 16 from the tooth roots part 18.