To provide a diaphragm actuator inhibiting response delay of an exhaust bypass valve.

This actuator includes a first shaft 2 of which one end an exhaust gas bypass valve 37 is connected to, of which the other end a first diaphragm 3 is fixed on, and which fully opens the exhaust bypass valve 37 when prescribed pressure is applied on the first diaphragm 3, and a second shaft 6 of which one end the first shaft 2 is engaged with, of which the other end a second diaphragm 5 on which supercharging pressure of a high pressure stage turbine 26 is applied is fixed to, and which operates the exhaust gas bypass valve 37 to an open side through the first shaft 2 according to supercharging pressure applied on the second diaphragm 5.