To suppress flame or toxic gas from flowing through a through opening formed in the wall to the next room side, and to enhance soundproof performance.

The electric connection box comprises a wiring box 2 being arranged on the back side of a wall W while facing a through opening H formed in the wall W, and an incidental connector 1 being connected with a cable led out from the wiring box 2 side. It is also provided with a frame 4 being secured to the wiring box 2 under a state located on the surface side of the wall W, and a box cover 6 made of thermally expandable heat resistant resin and substantially closing the through opening H through thermally expansion is interposed between the wiring box 2 and the frame 4. An annular portion 6a abutting against the periphery of the through opening H, and a recess 6B receiving the incidental connector 1 of the frame 4 are formed in the box cover.