The plug connector includes contacts and a block. The contacts are arranged on both sides of the block, and the block is formed at longitudinal ends with projection walls. The receptacle connector includes contacts and a housing. The contacts are arranged on both sides of the block to be opposite to each other in the width direction, and the housing is provided with engaging protrusions engaging the anchoring recess. The housing is further provided between the contacts longitudinally arranged with side walls A higher than upper surfaces of elastic portions of the contacts and with side walls B continuous with the side walls A and higher than upper surfaces of contact portions of the contacts. The plug contacts and the receptacle contacts are brought into contact with each other only after the plug connector and the receptacle connector have been completely fitted with each other.