An ignition device for an internal combustion engine has a spark plug, an ignition coil, a housing and a mounting portion. The spark plug is for discharging a current to ignite an air-fuel mixture in a combustion chamber of the internal combustion engine. The ignition coil is for supplying the current to the spark plug. The housing encloses the spark plug and the ignition coil therein. The housing has a mounting portion to be disposed in a mounting hole provided in a cylinder head of the internal combustion engine. The mounting portion includes a screw portion a non-screw portion. The screw portion is to be screw-fastened to the mounting hole. The non-screw portion is not to be screw-fastened to the mounting hole.