The invention relates to a connector and an assembly method of the connector, and aims at improving and designing operability in the preliminary stage of a connection operation in the connector used for preventing forced connection of a housing. A restriction device (30) used for preventing a second housing (20) from being fit in an incorrect oblique posture into a receptacle (12) comprises restricting ribs (31) projecting from an inner surface of the receptacle (12) and restricting grooves (32) formed by recessing an outer surface of the second housing (20). Each restricting rib (31) comprises a first rib (31A) extending in a connecting direction and second ribs (31B) located behind the front end of the first rib (31A) in the connecting direction. Each restricting groove (32) comprises afirst groove (32A) extending in the connecting direction and second grooves (32B) located before the rear end of the first groove (32A) in the connecting direction.