To improve response of a low-pressure stage turbine, by preventing excessive overshoot of engine exhaust pressure to prevent exhaust gas leakage when a load is abruptly changed, and guiding available exhaust gas in a high-pressure stage turbine to the low-pressure stage turbine.

A two-stage supercharging system comprises: a high-pressure stage turbocharger 2 and a low-pressure stage turbocharger 3 disposed in series to an exhaust passage 5 and an intake passage 6 of an engine 4; a first bypass passage 20 disposed to the exhaust passage 5 and bypassing the high-pressure stage turbine HT; a flow control valve 21 disposed to the first bypass passage 20; a second bypass passage 22 disposed to the exhaust passage 5 and bypassing the high-pressure stage turbine HT; and a relief valve 23 disposed to the second bypass passage 22, operated by the exhaust pressure upstream of the high-pressure stage turbine HT, and opened only when the exhaust pressure is increased to a predetermined value or more.