An electrical connection box includes a housing including at least a first and a second divided housings having annular peripheral edge portions of openings formed by outer peripheral walls that are fitted with each other. The first divided housing includes a first assembly member having a first outer wall body in which a cutout portion is formed, a second assembly member embedding the cutout portion and having a second outer wall body forming the outer peripheral wall 11 together with the first outer wall body, and a lock structure holding the first assembly member and the second assembly member in a completely assembled state. A first engagement body of the lock structure is provided on the first assembly member on the inner side relative to the cutout portion, and a second engagement body is provided on the second outer wall body on the inner wall surface side.