A spark-ignition engine comprises a plug hole (13) formed in a cylinder head (2), an ignition plug (10) and a plug cap (9) of the type integral with an ignition coil, which is attached to the ignition plug (10) as it is inserted into the plug hole (13). In this spark-ignition engine, the plug hole (13) is arranged at a position outside a head cover (5) in the cylinder head (2), a head surface (2a) of which is opened to provide the plug hole (13) and has a plug-cap attaching flange (11) attached thereto, and the plug-cap attaching flange (11) is formed with an extending cylinder-portion (15A) having an extension-hole (15) communicated with the plug hole (13). The plug cap (9), as it is inserted from the extension hole (15A) into the plug hole (13), has a coil portion (9C) fixed to the extending cylinder-portion (15).