A mechanical key for accommodation in a portable device for use in an electronic key system. The mechanical key is provided with a grip including a socket and a key plate fixed to the grip. The key plate includes a blade and a shank, which is inserted into the socket. The shank includes a first edge for abutting against the socket when the shank is inserted into the socket and a shoulder extending from the blade. The socket includes a guide for abutting against and guiding the first edge of the shank when the shank is inserted into the socket. The guide is mated with the first edge under pressure. The socket further includes a second edge for abutting against the shoulder when the shank is inserted into the socket. The second edge is mated with the shoulder under pressure.