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 perform a main ignition for igniting a mixture gas inside the main combustion chamber; a subspark plug configured to perform a subignition for igniting the mixture gas inside the subchamber; a throttle valve configured to change an air-fuel ratio of the mixture gas inside the main combustion chamber; 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 throttle valve, the control device being configured to: when an engine operates in a given first range, suspend the main ignition and the subignition to carry out compression self-ignition combustion of the mixture gas inside the main combustion chamber, and control the throttle valve so that the air-fuel ratio inside the main combustion chamber becomes a given first air-fuel ratio; when the engine operates in a second range adjacent to the first range, control at least one of the main spark plug and the subspark plug to perform the ignition to carry out flame propagation combustion of the mixture gas inside the main combustion chamber, and control the throttle valve so that the air-fuel ratio inside the main combustion chamber becomes a second air-fuel ratio lower than the first air-fuel ratio; and after a transition from the first range to the second range, control only the subspark plug to perform the ignition, or control the subspark plug and the main spark plug to perform the ignitions and set a timing of the main ignition to a timing same as or retarded from the subignition. 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 perform a main ignition for igniting a mixture gas inside the main combustion chamber; a subspark plug configured to perform a subignition for igniting the mixture gas inside the subchamber; a throttle valve configured to change an air-fuel ratio of the mixture gas inside the main combustion chamber; 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 throttle valve, the control device being configured to: when an engine operates in a given first range, suspend the main ignition and the subignition to carry out compression self-ignition combustion of the mixture gas inside the main combustion chamber, and control the throttle valve so that the air-fuel ratio inside the main combustion chamber becomes a given first air-fuel ratio; when the engine operates in a second range adjacent to the first range, control at least one of the main spark plug and the subspark plug to perform the ignition to carry out flame propagation combustion of the mixture gas inside the main combustion chamber, and control the throttle valve so that the air-fuel ratio inside the main combustion chamber becomes a second air-fuel ratio lower than the first air-fuel ratio; and after a transition from the first range to the second range, control only the subspark plug to perform the ignition, or control the subspark plug and the main spark plug to perform the ignitions and set a timing of the main ignition to a timing same as or retarded from the subignition. when an engine operates in a given first range, suspend the main ignition and the subignition to carry out compression self-ignition combustion of the mixture gas inside the main combustion chamber, and control the throttle valve so that the air-fuel ratio inside the main combustion chamber becomes a given first air-fuel ratio; when the engine operates in a second range adjacent to the first range, control at least one of the main spark plug and the subspark plug to perform the ignition to carry out flame propagation combustion of the mixture gas inside the main combustion chamber, and control the throttle valve so that the air-fuel ratio inside the main combustion chamber becomes a second air-fuel ratio lower than the first air-fuel ratio; and after a transition from the first range to the second range, control only the subspark plug to perform the ignition, or control the subspark plug and the main spark plug to perform the ignitions and set a timing of the main ignition to a timing same as or retarded from the subignition.