A modular jack (31) comprises a housing (32) and a foolproof mechanism (34) including a pair of spring members (42) cantilevered to the housing (32), and an abutment section (43) provided at the front end of each spring member (42). Each abutment section (43) consists of a cam portion (45) having an inclined face (47) and a stopper portion (44) provided behind and inside the cam portion (45). When a modular plug (2) having a predetermined number of poles is inserted into the modular jack (31), the front ends of the modular plug (2) abut the inclined faces (47) of the cam portions (45) to move the abutment sections (43) outwardly, permitting insertion of the modular plug (2). When a modular plug (2) having a number of poles that is smaller than the predetermined number is inserted, the front ends of the modular plug (2) abut against the stopper portions (44) to block insertion of the modular plug (2).