The invention provides a vertical internal combustion engine with overhead valves wherein a crankshaft is vertically arranged and the cylinder is horizontally arranged. A carburetor is arranged on one side of the cylinder and a muffler is arranged on another side of the cylinder, opposite the carburetor. A suction port is connected to the carburetor by a horizontal straight line portion and a curved portion, leading from the horizontal straight line portion to a suction valve. The horizontal straight line portion includes a center line and the carburetor includes a center which is substantially aligned with the center line of the horizontal straight line portion, along a straight line. The exhaust port is connected to the muffler by an exhaust passage having a center line which passes through the center of the exhaust port and crosses the center line of the horizontal straight line portion, diagonally from below. According to another aspect of the invention, the suction port is connected to the carburetor by a horizontal straight line portion and a curved portion, leading to a suction valve, wherein the center line of the horizontal straight portion and the center of the carburetor are aligned and the curved portion leads smoothly from an end of the horizontal straight line portion to the suction valve.