A control system for a compression-ignition engine, comprising: a combustion chamber of the engine defined by a cylinder, a piston configured to reciprocate in the cylinder, and a cylinder head closing one end of the cylinder; an injector attached to the cylinder head and configured to inject fuel to be supplied into the combustion chamber; an ignition plug disposed so as to be oriented in the combustion chamber and configured to ignite a mixture gas inside the combustion chamber; at least one sensor device (SW1-SW17) configured to measure at least one parameter related to operation of the engine; and a controller having circuity connected with the ignition plug, the injector, and the at least one sensor device (SW1-SW17), and configured to perform a calculation in response to the measurement signal from the sensor device (SW1-SW17) and output a signal to the ignition plug and the injector, wherein the ignition plug is configured to forcibly ignite the mixture gas to start combustion accompanied by flame propagation of a portion of the mixture gas, and wherein the remaining unburnt mixture gas combusts by self-ignition, wherein the controller is configured to execute: an ignition controlling module configured to output an ignition signal to the ignition plug before a target timing so that the unburnt mixture gas self-ignites at the target timing; an ignition timing estimating module (101o, 101p) configured to estimate an actual compression ignition timing indicative of a timing at which the unburnt mixture gas actually self-ignites based on an in-cylinder pressure parameter related to a pressure inside the combustion chamber measured by the sensor device (SW1-SW17); and an in-cylinder temperature determining module (101α) configured to determine the in-cylinder temperature at a given crank angle based on the estimated result by the ignition timing estimating module (101o, 101p). characterized in that the controller includes a spark ignition timing calculating module configured to calculate a crank angle at which a combustion mass ratio becomes a given value based on the parameter measured by the sensor device (SW1-SW17), and the in-cylinder temperature determining module (101α) is configured to determine the in-cylinder temperature based on the actual compression ignition timing and the crank angle calculated by the spark ignition timing calculating module. a combustion chamber of the engine defined by a cylinder, a piston configured to reciprocate in the cylinder, and a cylinder head closing one end of the cylinder; an injector attached to the cylinder head and configured to inject fuel to be supplied into the combustion chamber; an ignition plug disposed so as to be oriented in the combustion chamber and configured to ignite a mixture gas inside the combustion chamber; at least one sensor device (SW1-SW17) configured to measure at least one parameter related to operation of the engine; and a controller having circuity connected with the ignition plug, the injector, and the at least one sensor device (SW1-SW17), and configured to perform a calculation in response to the measurement signal from the sensor device (SW1-SW17) and output a signal to the ignition plug and the injector, wherein the ignition plug is configured to forcibly ignite the mixture gas to start combustion accompanied by flame propagation of a portion of the mixture gas, and wherein the remaining unburnt mixture gas combusts by self-ignition, wherein the controller is configured to execute: an ignition controlling module configured to output an ignition signal to the ignition plug before a target timing so that the unburnt mixture gas self-ignites at the target timing; an ignition timing estimating module (101o, 101p) configured to estimate an actual compression ignition timing indicative of a timing at which the unburnt mixture gas actually self-ignites based on an in-cylinder pressure parameter related to a pressure inside the combustion chamber measured by the sensor device (SW1-SW17); and an in-cylinder temperature determining module (101α) configured to determine the in-cylinder temperature at a given crank angle based on the estimated result by the ignition timing estimating module (101o, 101p). an ignition controlling module configured to output an ignition signal to the ignition plug before a target timing so that the unburnt mixture gas self-ignites at the target timing; an ignition timing estimating module (101o, 101p) configured to estimate an actual compression ignition timing indicative of a timing at which the unburnt mixture gas actually self-ignites based on an in-cylinder pressure parameter related to a pressure inside the combustion chamber measured by the sensor device (SW1-SW17); and an in-cylinder temperature determining module (101α) configured to determine the in-cylinder temperature at a given crank angle based on the estimated result by the ignition timing estimating module (101o, 101p). characterized in that the controller includes a spark ignition timing calculating module configured to calculate a crank angle at which a combustion mass ratio becomes a given value based on the parameter measured by the sensor device (SW1-SW17), and the in-cylinder temperature determining module (101α) is configured to determine the in-cylinder temperature based on the actual compression ignition timing and the crank angle calculated by the spark ignition timing calculating module.