A housing structure includes: a first housing having a fitting inner tubular portion that has an inner tubular end surface and a first bolt fastening portion that has a first bolt aligning surface formed at a position away from the inner tubular end surface in the radial direction, the first housing supporting a first end of an assist shaft via a first bearing; and a second housing having a fitting outer tubular portion having an outer tubular end surface and a second bolt fastening portion that has a second bolt aligning surface that mates with the first bolt aligning surface, the second housing supporting a second end of the assist shaft via a second bearing. A first gap is provided between the inner tubular end surface and an inner tubular opposite surface, and a second gap is provided between the outer tubular end surface and an outer tubular opposite surface.