Provided is a casing structure including: an annular seal member which is disposed at a connection portion between first and second casings to keep air-tightness between an internal space formed by connecting the first and second casings to each other and an outside of the internal space; a first screw member which is threaded from the internal space, is disposed at the inside of the seal member in the radial direction, and fastens the first and second casings to each other; and a second screw member which is threaded from the outside, is disposed at the outside of the seal member in the radial direction, and fastens the first and second casings to each other.