A transmission mechanism used for sewing machines equipped with a carriage comprising a base plate having an opening, a fixing clamp for mounting the base plate to the carriage, and an apparatus for transmission which can be vertically attached to the base plate under the opening thereof. The transmission apparatus has a plunger passable through the opening to perform a reciprocating movement. The transmission apparatus has all its elements substantially disposed within a housing encased by an upper cover and a lower cover thereof to form an integral piece and to reduce the total length thereof.