To provide a diaphragm actuator which, as compared with conventional ones, allows easier selection of a biasing member for applying a biasing force to a displacement element regardless of how much the displacement element is displaced.

The diaphragm actuator has: a housing 41 separated into a first chamber C1, a second chamber C2 and a third chamber C3; a first diaphragm 45 for separating the first chamber C1 from the second chamber C2; a second diaphragm 49 for separating the second chamber C2 from the third chamber C3; an incompressible fluid sealed in the second chamber C2; and a displacement member connected to the second diaphragm 49 and displaced according to the internal pressure difference between the first chamber C1 and the third chamber C3. The area of the pressure-receiving surface of the first diaphragm 45 is set to be larger than that of the pressure-receiving surface of the second diaphragm 49. A biasing member for biasing the first diaphragm 45 is provided inside the housing 41. The biasing member has a biasing force to separate the first diaphragm 45 from the second diaphragm 49.