A housing member for a power transmission device connected to a driving source via a torque converter housed in the housing member includes a torque converter side mating face closer to the torque converter. The torque converter side mating face has a single-wall structure and a double-wall structure, wherein two adjacent fastening regions are connected circumferentially by a wall in the single-wall structure, and wherein two adjacent fastening regions are connected circumferentially by each of two walls in the double-wall structure. The torque converter side mating face includes: an upper half region in which the double-wall structure has a greater proportion in area than the single-wall structure; and a lower half region in which the double-wall structure has a less proportion in area than the single-wall structure.