Ignition system

An ignition system (101) includes an ignition plug (1) having a spark discharge gap (33) formed between a center electrode (5) and a ground electrode (27), and a power supply (51) for supplying electric energy to the spark discharge gap (33). Spark discharge is produced when electric energy is supplied from the power supply (51) to the spark discharge gap (33). The electric energy output from the power supply (51) for producing spark discharge of one unit is set to 100 mJ or greater. S1 ≥ [{-30(mm)×G1+60}/100]×S2 and G1 < 2.0 are satisfied wherein G1 represents the size (mm) of the spark discharge gap (33), and S1 and S2 represent areas (mm) defined such that when the center electrode (5) and the ground electrode (27) are projected on a plane VS orthogonal to the axis CL1, a region obtained by removing, from a projection region (5P) of the center electrode (5), a region where the projection region (5P) overlaps with a projection region (27P) of the ground electrode (27) has the area S1, and the projection region (5P) of the center electrode (5) has the area S2.