An engine has a crankcase, and a cylinder which extends from an upper portion of the crankcase toward a front side. In a space which is formed at an upper side of a horizontal plane which substantially horizontally extends from a bottom surface of the crankcase, a catalyst storage chamber is arranged in such a manner that an axis of the catalyst converter is in a direction of a crankshaft. The space is arranged just below a front surface of the cylinder and forward of a front surface of the crankcase.