To provide a partition frame mounting structure easy to position and superior in mounting strength.

A peripheral face 21 of a front face opening portion 20 is formed by connecting flanges 11A, 12A protruded from the opening edges of an outer box 11 and an inner box 12 to each other via a synthetic resin connection frame 25. The connection frame 25 is parted at its longitudinal center to form a preset-width mounting groove 45 between both parted end faces 26A of the frame body 26. A partition frame 30 has a heat insulating material 70 arranged on the rear face side of a metal front face frame 50 having mounting plates 55 on both end faces and a synthetic resin cover 60 applied thereon. The end of a side face plate 52 of the front face frame 50 is protruded from the end of a side face plate 62 of the cover 60 to form a stepped portion 90. The stepped portion 90 is tightly fitted into the mounting groove 45. The front face frame 50 with its end fitted into the mounting groove 45 is accurately positioned. The rotation is prevented to actualize a strong mounting structure resisting against impact.