An intake pipe structure of an engine comprises: a joint pipe unit constituted by a return pipe portion and an intake pipe portion for introducing air into the engine. The return pipe portion has one end connected with the intake pipe portion to return blow-by gas from the engine to the intake pipe portion. The return pipe portion has first and second surface sections respectively extending in the range of first and second lengths from the one end of the return pipe portion, the first surface section smoothly and continuously extending, and the second surface section being shorter than the first surface section. The return pipe portion has a curved surface portion enlarged and curved at its downstream side toward the one end of the return pipe portion, and an opening opened at the one end to the intake pipe portion in the form of non-circular shape.