To reduce workload in case of assembling after manufacturing a lower frame in a state of being divided, and strengthen the monobloc property in a state of being assembled.

An outdoor side lower frame material 2 having an upper depth piece 21 and a lower depth piece 23, and an indoor side lower frame material 3 having an upper depth piece 21 and a lower depth piece 33, are assembled in the depth direction, and the outdoor side lower frame material 2 and the indoor side lower frame material 3 are connected together so that the upper depth pieces 21 and the lower depth pieces 23, 33 are respectively connected with each other. The one upper depth piece 21 is formed with a projection 22 projecting downward, meanwhile, the other upper depth piece 21 is formed with a fitting groove 32 on the position corresponding to the projection 22, the projection 22 is fitted to the fitting groove 32 so as to connected the upper depth pieces 21 with each other, and the lower depth pieces 23, 33 are overlapped with each other so as to connected together by screws 5.