A control system for an engine, the engine having a combustion chamber, an injector, and a spark plug, the combustion chamber being formed by a cylinder, a piston and a cylinder head, the injector being attached to the engine and configured to supply fuel into the combustion chamber, the spark plug being disposed to be oriented into the combustion chamber, the control system comprising: an adjuster (13a, 14a, 53) configured to change an EGR ratio that is a ratio of burned gas introduced into and/or remained in the combustion chamber, the adjuster (13a, 14a, 53) being configured to change the EGR ratio so that a compression start temperature of the combustion chamber rises as an engine load is reduced; and a controller to be connected to the injector and the spark plug, and configured to output a control instruction to the injector and the spark plug, respectively, wherein the controller is configured to output an ignition instruction to the spark plug so as to ignite after the EGR ratio adjustment, so that partial compression-ignition combustion in which a mixture gas combusts by flame propagation and then combusts by compression ignition is performed, the adjuster (13a, 14a) is a variable valve mechanism configured to adjust an internal EGR ratio, that is a ratio of residual burned gas in the combustion chamber, by changing open and close timings, or open and close crank angles, of at least one of an intake valve and an exhaust valve, the controller is configured to control, during the partial compression-ignition combustion, the variable valve mechanism (13a, 14a) to increase the internal EGR ratio as the engine load is reduced, the variable valve mechanism (13a, 14a) is configured to adjust an exhaust opened state extending period that is a period during which the exhaust valve opens after an exhaust top dead center, the controller is configured to control, during the partial compression-ignition combustion, the variable valve mechanism (13a, 14a) to extend the exhaust opened state extending period as the engine load is reduced, and the control system further comprises an recirculation device configured to introduce into the combustion chamber a portion of burned gas generated inside the combustion chamber, characterized in that the controller is configured to control, during the partial compression-ignition combustion, the variable valve mechanism (13a, 14a) to retard the close timing, or close crank angle, of the intake valve with respect to an intake bottom dead center as the engine load is reduced. an adjuster (13a, 14a, 53) configured to change an EGR ratio that is a ratio of burned gas introduced into and/or remained in the combustion chamber, the adjuster (13a, 14a, 53) being configured to change the EGR ratio so that a compression start temperature of the combustion chamber rises as an engine load is reduced; and a controller to be connected to the injector and the spark plug, and configured to output a control instruction to the injector and the spark plug, respectively, wherein the controller is configured to output an ignition instruction to the spark plug so as to ignite after the EGR ratio adjustment, so that partial compression-ignition combustion in which a mixture gas combusts by flame propagation and then combusts by compression ignition is performed, the adjuster (13a, 14a) is a variable valve mechanism configured to adjust an internal EGR ratio, that is a ratio of residual burned gas in the combustion chamber, by changing open and close timings, or open and close crank angles, of at least one of an intake valve and an exhaust valve, the controller is configured to control, during the partial compression-ignition combustion, the variable valve mechanism (13a, 14a) to increase the internal EGR ratio as the engine load is reduced, the variable valve mechanism (13a, 14a) is configured to adjust an exhaust opened state extending period that is a period during which the exhaust valve opens after an exhaust top dead center, the controller is configured to control, during the partial compression-ignition combustion, the variable valve mechanism (13a, 14a) to extend the exhaust opened state extending period as the engine load is reduced, and the control system further comprises an recirculation device configured to introduce into the combustion chamber a portion of burned gas generated inside the combustion chamber, characterized in that the controller is configured to control, during the partial compression-ignition combustion, the variable valve mechanism (13a, 14a) to retard the close timing, or close crank angle, of the intake valve with respect to an intake bottom dead center as the engine load is reduced.