An ignition system comprising: a dividing wall which divides a combustion chamber of an engine into a main chamber and a prechamber and has formed therein at least one spray hole which communicates between the main chamber and the pre-chamber; and a spark plug in which voltage is applied across a spark gap between a first electrode and a second electrode to create an electrical spark ( f) to ignite fuel, wherein the pre-chamber houses the spark plug with the first electrode, the dividing wall or a member which is electrically connected to the dividing wall has the second electrode, an after-top-dead-center ignition control mode in which an ignition operation to ignite the fuel using the spark plug is performed after a compression stroke top dead center ( Td) is executed when a given operating condition of the engine is met, and in the after-top-dead-center ignition control mode, an ignition source which is in a form of a self-growable flame kernel is provided in a spray hole-nearby region within the pre-chamber, the spray hole, or the main chamber within a crank angle of 20° after an ignition timing ( Ts) where the ignition timing ( Ts) is a timing when the voltage starts to be applied across the spark gap, and the spray hole-nearby region is a region which is located 3mm or less away from a spray hole center that is a center of an opening of the spray hole which faces the pre-chamber, characterized in that a relation of S / V ≤ -0.025D^3 + 0.34D^2 - 1.4D + 2.1 is met where V is a volume of the pre-chamber in units of cubic centimeter, S is a total sectional area of the at least one spray hole in the dividing wall in units of mm^2, and D is a distance between the first electrode and spray hole in units of mm or when the dividing wall has a plurality of spray holes formed therein, D denotes a distance between the first electrode and one of the spray holes which is located closest to the spark gap. a dividing wall which divides a combustion chamber of an engine into a main chamber and a prechamber and has formed therein at least one spray hole which communicates between the main chamber and the pre-chamber; and a spark plug in which voltage is applied across a spark gap between a first electrode and a second electrode to create an electrical spark ( f) to ignite fuel, wherein the pre-chamber houses the spark plug with the first electrode, the dividing wall or a member which is electrically connected to the dividing wall has the second electrode, an after-top-dead-center ignition control mode in which an ignition operation to ignite the fuel using the spark plug is performed after a compression stroke top dead center ( Td) is executed when a given operating condition of the engine is met, and in the after-top-dead-center ignition control mode, an ignition source which is in a form of a self-growable flame kernel is provided in a spray hole-nearby region within the pre-chamber, the spray hole, or the main chamber within a crank angle of 20° after an ignition timing ( Ts) where the ignition timing ( Ts) is a timing when the voltage starts to be applied across the spark gap, and the spray hole-nearby region is a region which is located 3mm or less away from a spray hole center that is a center of an opening of the spray hole which faces the pre-chamber, characterized in that a relation of S / V ≤ -0.025D^3 + 0.34D^2 - 1.4D + 2.1 is met where V is a volume of the pre-chamber in units of cubic centimeter, S is a total sectional area of the at least one spray hole in the dividing wall in units of mm^2, and D is a distance between the first electrode and spray hole in units of mm or when the dividing wall has a plurality of spray holes formed therein, D denotes a distance between the first electrode and one of the spray holes which is located closest to the spark gap.