An electrical junction box, which is assembled by first inserting a circuit board assembly into a primary housing via an opening of the primary housing, and after that inserting a secondary housing into the primary housing, includes a connector block that has a projection that widens the opening that has been narrowed. A plurality of ribs are formed on front and rear walls of the thin primary housing from the side of the opening in an insertion direction of the circuit board assembly. A concave groove formed between a pair of the projections engages with the rib of the front wall. The ribs on the rear wall are close to a circuit board of the circuit board assembly. The connector block has protuberances that abut against the ribs.