In a shaft body, a first end in an axial direction of a core part having a crisscross cross section orthogonal to the axial direction is connected to a helical gear via a first connecting part, and the second end in the axial direction of the core part is connected to a worm via a second connecting part. A plurality of first framework parts are formed at regular intervals along a direction of a shaft core in the core part between the first connecting part and the second connecting part. A second framework part extends across the first connecting part, the first framework part, and the core part like a diagonal brace. A third framework part extends across a pair of the first framework parts adjacent to each other and the core part like a diagonal brace. A fourth framework part extends across the second connecting part, the first framework part, and the core part like a diagonal brace.