A fixing structure for an electrical junction box and a cassette, the fixing structure being adapted to fix the cassette on a side wall outer surface of the electrical junction box. A locking portion is provided on an outer surface of the cassette. A rib plate that extends in a vertical direction and projects downward adjacent to the locking portion to come into contact with the side wall outer surface. A counterpart locking portion to be locked on the locking portion is provided on the side wall outer surface. An engaging stepped-portion receives a lower end of the rib plate. When the locking portion and the counterpart locking portion are coupled to and locked on one another, the rib plate comes into contact with the side wall outer surface, and a lower end of the rib plate comes into contact with the engaging stepped-portion to restrict rotation of the cassette.