A casing assembly, into which a rotor rotatable around an axis is inserted, includes: a lower half casing including a lower half flange surface and a lower half accommodation recessed portion; an upper half casing including an upper half flange surface and an upper half accommodation recessed portion; a fixing portion that fixes the lower half casing and the upper half casing to each other such that, when the lower and the upper half flange surfaces make contact, the lower half accommodation recessed portion and the upper half accommodation recessed portion form an accommodation space extending about the axis; a housing disposed in the accommodation space; and a seal member disposed on an outer peripheral surface of the housing and in contact with an inner peripheral surface of the lower half accommodation recessed portion and an inner peripheral surface of the upper half accommodation recessed portion.