An ignition system 100 includes an ignition plug 1, a discharge power supply 2 for applying a high voltage to the gap 19 of the ignition plug 1, a high-frequency power supply 3 for supplying a high-frequency current to the gap 19, a matching unit 4 provided between the ignition plug 1 and the high-frequency power supply 3, and a mixer 5 through which currents output from the two power supplies 2, 3 flow. An oscillation frequency fs (Hz) which maximizes the current flowing between the matching unit 4 and the mixer 5 when spark discharge is generated and an oscillation frequency fo (Hz) which maximizes the current flowing between the matching unit 4 and the mixer 5 when spark discharge is not generated satisfy a relation fs/fo ≥ 0.85. This allows a current whose oscillation frequency is equal to the oscillation frequency fs to flow between the matching unit 4 and the mixer 5 when spark discharge is not generated.