To quickly assemble a governor shaft in an engine body by a short installing-moving stroke.

The governor shaft 19 driven by a governor sleeve of a mechanical governor is rotatably incorporated into the engine body, and the governor shaft 19 is connected to a throttle valve of a carburetor via a governor lever. The engine body is provided with a main bearing 31 for supporting a base end part 19a of the governor shaft 19, and is provided with an auxiliary bearing 32 for supporting a central part of the governor shaft 19. An opening part 37 is formed in the auxiliary bearing 32 by communicating with a bearing hole 36 of this bearing, and a constricted part 38 is formed on the governor shaft 19, and this constricted part 38 is inserted into the bearing hole 36 via the opening part 37.