To provide an electric connection box fittable to a wall easily and surely.

This box comprizes a casing 10 for housing a receptacle, a switch, and so on, to be put on the rear side of a wall W of a building structure interposing an approximately rectangular through hole bored in the wall W, flanges provided on surfaces opposite to each other of the casing 10 respectively and being in contact with the rear of the wall W, and a fitting board 20 being approximately in a rectangular shape, fitted with a receptacle, a switch, and so on, being opposed to the casing 10, and to be put on the front side of the wall W and fixed to this casing 10, and houses a plug socket and so on. In this case, the casing 10 is approximately in the shape of a closed-end box and has an opening, and electric wires are taken into the casing 10 from the bottom.