A two-stage turbo system provided with an internal combustion engine includes, two turbo-superchargers driven by exhaust gas from the engine, control valves for switching between the flow path of intake gas sucked into the engine and the flow path of exhaust gas, and a control device for controlling the control valves and the turbo-superchargers. The two turbo-superchargers have the same turbine capacity, function respectively as the high-pressure side turbo-supercharger on the exhaust path upstream side and the low-pressure side turbo-supercharger on the exhaust path downstream side, and have, by switching between the flow paths by using the control valves, a series mode in which the two turbo-superchargers are serially connected to each other, a one-stage supercharging mode in which gas flows only to the high-pressure-side turbo-supercharger or only to the low-pressure-side turbo-supercharger, and a parallel mode in which the two turbo-superchargers are connected in parallel to each other.