Abstract: To reduce number of parts and reduce cost in a multi-pin modular jack provided with a mis-insertion prevention mechanism that prevents insertion of a modular plug having number of pins different from the modular jack the mis-insertion prevention mechanism includes a mis-insertion determination unit having a mis-insertion prevention guide mounted at the front that causes a modular plug having different number of pins from the modular jack inserted into an opening in a housing of the modular jack to move traversely and an elastic support mounted at the rear and fixedly supported at the rear of the housing, and a stopper formed on a portion of the roof of the housing on a side opposite the side on which the mis-insertion determination unit. The mis-insertion determination unit is disposed on either an upper left or an upper right top part inside the opening in the housing.