An ignition device performs spark ignition to a fuel mixture in a combustion chamber (13) of an internal combustion engine (100, 101) by using a spark plug (50). The spark plug (50) includes a first electrode (51), a second electrode (52, 11a, 11b, 21), and an insulating member (53, 11c) which is formed from dielectric substance and interposed between the first electrode (51) and the second electrode (52, 11a, 21). By impressing an alternating current between the first electrode (51) and the second electrode (52, 11a, 21), non-equilibrium plasma discharge between the insulating member (53, 11c) and one of the first electrode (51) and the second electrode (52, 11a, 21) is promoted. Igniting the fuel mixture by the non-equilibrium plasma discharge achieves a high ignition performance is achieved with low energy consumption.