To prevent a seal member from coming off.

An ignition device 10 for an internal combustion engine, partly received in a plug hole 3a of a plug tube 3, is constituted of a coil main unit upper part 10a, coil main unit lower part 10b, connection part 10c, fixing bracket 11, connector part 12, control circuit part and a packing 6. The packing 6 is mounted in a side of a head cover 2, formed annularly in a U-shaped section so as to interpose upper/lower surfaces of a flange part 2c of the head cover 2, and shape of the packing 6 is formed to be matched with a small/large diametric hole constituting a hole part of the head cover 2. In this way, moving to the outside of the head cover 2 of the packing 6 is impeded, so that coming-off of the packing 6 from the head cover 2 can be prevented when an ignition coil, comprising the coil main unit upper part 10a, coil main unit lower part 10b, or the like is mounted/demounted.