A receptacle connector (3) includes a receptacle contact (7) corresponding to a plug contact (4), a receptacle housing (8) that holds the receptacle contact (7), and an assistant fitting (9) attached to the receptacle housing (8), and is mounted on a connector mounting surface (10a) of a circuit board (10). The assistant fitting (9) includes at least a held portion (30), a fixing portion (31), and a vertical displacement regulating portion (42). The held portion (30) is a portion that is held by the receptacle housing (8). The fixing portion (31) is a portion that is hooked to the plug housing (5) to thereby fix the plug connector (2) to the receptacle connector (3). The vertical displacement regulating portion (42) is a portion that regulates a displacement of the fixing portion (31) in a direction away from the connector mounting surface (10a) of the circuit board (10).