Provided is an assembly structure of an electrical junction box which makes it possible to fit an electrical component attachment block into an insulation case without provision of a lock mechanism. Included are a housing (15) provided in an insulation case (5) and configured to house an electrical component attachment block (7); contact walls (19, 20) provided in an inner wall of the housing (15), and configured to come into contact with the housed electrical component attachment block (7); a terminal receiving portion (23) provided to the insulation case (5), on which to place a connection terminal (8); and a fastening module (9) configure to screw-fix the connection terminal (8) placed on the terminal receiving portion (23) to a conduction bus bar (37) in the electrical component attachment block (7) housed in the housing (15). The electrical component attachment block (7) is fitted into the insulation case (5) by fixing the connection terminal (8) to the conductor bus bar (37) by use of the fastening module (9) with the electrical component attachment block (7) put in contact with the contact walls (19, 20).