It is an object of the present invention to provide an engine which can easily be produced. In an engine including an electronic throttle device, a control unit for controlling the electronic throttle device, and a gas fuel supply unit, a gaseous-fuel mixer is used as the gas fuel supply unit, and when an extending direction of a crankshaft is defined as a longitudinal direction and an engine cooling fan is defined as existing on a front side and a side opposite from the front side is defined as a rear side, a collector of an intake manifold extends in the longitudinal direction, the electronic throttle device is mounted on the rear side of the collector, and the gaseous-fuel mixer is mounted on the rear side of the electronic throttle device.