There is provided a spark-ignition engine with which a gas supply device and an intake system can be integrated with the engine. In the spark-ignition engine including: a throttle body; a gas mixer; a gas regulator; and a gas relay pipe, a collector portion of an intake manifold is supported to extend in a front-back direction on one side of a cylinder head cover, the throttle body is mounted to a back portion of the collector portion of the intake manifold, the gas mixer is mounted to a back portion of the throttle body above a flywheel, the gas regulator is disposed on a side of an exhaust manifold above the flywheel and mounted to a cylinder head, and the gas relay pipe is supported to extend sideways between the gas regulator and the gas mixer behind the cylinder head cover.