A cabin support deck for supporting a cabin of a construction machine on the underside of the cabin includes a frame member and a mounting portion. The frame member includes a proximal segment at which the frame member is supported, a distal segment, a left side segment, and a right side segment. The distal segment has a first structure having a first lateral dimension. The proximal segment has a second structure having a second lateral dimension which is greater than the first lateral dimension. At least one of the proximal side sections of the left side segment and the right side segment has the second structure, and the other portions of the left side segment and the right side segment have the first structure.