An engine system, comprising: a cylinder block and a cylinder head configured to form a cylinder; a piston reciprocatably accommodated in the cylinder; a main combustion chamber defined by the cylinder block, the cylinder head, and the piston; a subchamber that is separated from the main combustion chamber by a partition, and configured to communicate with the main combustion chamber through a communicating hole formed in the partition; an injector configured to inject fuel into the main combustion chamber; a main spark plug configured to ignite a mixture gas inside the main combustion chamber; a subspark plug configured to ignite the mixture gas inside the subchamber; an EGR device configured to recirculate part of exhaust gas discharged from the cylinder to an intake passage through which intake air introduced into the cylinder circulates; and a control device that is electrically connected to, and configured to output an electric signal for control to, the injector, the main spark plug, the subspark plug, and the EGR device, wherein when an engine is operated in a specific range where part of the exhaust gas is recirculated to the intake passage by the EGR device as EGR gas, the control device is configured to: control the main spark plug and the subspark plug so that a subignition timing (ts) that is an ignition timing of the subspark plug is retarded from a main ignition timing (tm) that is an ignition timing of the main spark plug, and control the main spark plug and the subspark plug so that an ignition phase difference (dt) that is a retard amount of the subignition timing (ts) with respect to the main ignition timing (tm) becomes larger under a first EGR condition than a second EGR condition, the first and second EGR conditions being conditions in the specific range in which engine speeds are the same and EGR rates are different, the EGR rates each being a ratio of EGR gas with respect to all gas existing in the main combustion chamber and the subchamber, and the first EGR condition having a greater EGR rate than the second EGR condition. a cylinder block and a cylinder head configured to form a cylinder; a piston reciprocatably accommodated in the cylinder; a main combustion chamber defined by the cylinder block, the cylinder head, and the piston; a subchamber that is separated from the main combustion chamber by a partition, and configured to communicate with the main combustion chamber through a communicating hole formed in the partition; an injector configured to inject fuel into the main combustion chamber; a main spark plug configured to ignite a mixture gas inside the main combustion chamber; a subspark plug configured to ignite the mixture gas inside the subchamber; an EGR device configured to recirculate part of exhaust gas discharged from the cylinder to an intake passage through which intake air introduced into the cylinder circulates; and a control device that is electrically connected to, and configured to output an electric signal for control to, the injector, the main spark plug, the subspark plug, and the EGR device, wherein when an engine is operated in a specific range where part of the exhaust gas is recirculated to the intake passage by the EGR device as EGR gas, the control device is configured to: control the main spark plug and the subspark plug so that a subignition timing (ts) that is an ignition timing of the subspark plug is retarded from a main ignition timing (tm) that is an ignition timing of the main spark plug, and control the main spark plug and the subspark plug so that an ignition phase difference (dt) that is a retard amount of the subignition timing (ts) with respect to the main ignition timing (tm) becomes larger under a first EGR condition than a second EGR condition, the first and second EGR conditions being conditions in the specific range in which engine speeds are the same and EGR rates are different, the EGR rates each being a ratio of EGR gas with respect to all gas existing in the main combustion chamber and the subchamber, and the first EGR condition having a greater EGR rate than the second EGR condition. control the main spark plug and the subspark plug so that a subignition timing (ts) that is an ignition timing of the subspark plug is retarded from a main ignition timing (tm) that is an ignition timing of the main spark plug, and control the main spark plug and the subspark plug so that an ignition phase difference (dt) that is a retard amount of the subignition timing (ts) with respect to the main ignition timing (tm) becomes larger under a first EGR condition than a second EGR condition, the first and second EGR conditions being conditions in the specific range in which engine speeds are the same and EGR rates are different, the EGR rates each being a ratio of EGR gas with respect to all gas existing in the main combustion chamber and the subchamber, and the first EGR condition having a greater EGR rate than the second EGR condition.