A knocking control system for a gas engine comprising: an ignition device for igniting an air-fuel mixture supplied to the gas engine ; a knocking detector for detecting a knocking in a cylinder of the gas engine ; a phase angle detector for detecting a phase angle of the gas engine ; and a controller for driving the ignition device , the controller being configured to calculate a knocking occurrence ratio (SEV) which is a ratio of a cycle number (N L ) of cycles in which a predetermined knocking occurs for a period of the cycles of the predetermined cycle number (N), to the predetermined cycle number (N), perform a delay calculation of the knocking occurrence ratio (SEV) to calculate an occurrence ratio delay calculation value (SEV AVE ), calculate a deviation (Δ SEV) between the occurrence ratio delay calculation value (SEV AVE ) and a predetermined occurrence ratio target value (SEV SET ), and drive the ignition device so that the ignition timing reaches a command value (IGN) decided according to a target value (IGN SET1 ) of the ignition timing of the ignition device wherein the target value (IGN SET1 ) is obtained by retarding a reference value (IGN AVE ) of the ignition timing of the ignition device based on the deviation (Δ SEV), if the occurrence ratio delay calculation value (SEV AVE ) is larger than the occurrence ratio target value (SEV SET ) and by putting the reference value (IGN AVE ) of the ignition timing of the ignition device advanced based on the deviation (Δ SEV), if the occurrence ratio delay calculation value (SEV AVE ) is smaller than the occurrence ratio target value (SEV SET ). an ignition device for igniting an air-fuel mixture supplied to the gas engine ; a knocking detector for detecting a knocking in a cylinder of the gas engine ; a phase angle detector for detecting a phase angle of the gas engine ; and a controller for driving the ignition device , the controller being configured to calculate a knocking occurrence ratio (SEV) which is a ratio of a cycle number (N L ) of cycles in which a predetermined knocking occurs for a period of the cycles of the predetermined cycle number (N), to the predetermined cycle number (N), perform a delay calculation of the knocking occurrence ratio (SEV) to calculate an occurrence ratio delay calculation value (SEV AVE ), calculate a deviation (Δ SEV) between the occurrence ratio delay calculation value (SEV AVE ) and a predetermined occurrence ratio target value (SEV SET ), and drive the ignition device so that the ignition timing reaches a command value (IGN) decided according to a target value (IGN SET1 ) of the ignition timing of the ignition device wherein the target value (IGN SET1 ) is obtained by retarding a reference value (IGN AVE ) of the ignition timing of the ignition device based on the deviation (Δ SEV), if the occurrence ratio delay calculation value (SEV AVE ) is larger than the occurrence ratio target value (SEV SET ) and by putting the reference value (IGN AVE ) of the ignition timing of the ignition device advanced based on the deviation (Δ SEV), if the occurrence ratio delay calculation value (SEV AVE ) is smaller than the occurrence ratio target value (SEV SET ). <EOS>