Provided is a technique of reducing a force required to couple a receptacle connector and a plug connector. A receptacle assistant fitting of the receptacle connector includes a cantilevered lock beam having a deflection part extending in a direction substantially parallel to a connector mounting surface of a receptacle side substrate and in the outside of a receptacle housing. A plug assistant fitting of the plug connector includes an engagement part that pushes aside the free end of the lock beam and rides over the free end of the lock beam so as to be engaged with the free end of the lock beam when the plug connector and the receptacle connector are coupled to each other.