To provide a differential-pressure detecting device which can be miniaturized easily and which is of high reliability and to provide a fluid-pressure actuator which uses the detecting device and which is of high reliability.

In this differential-pressure detecting device, a first pressure detector 11 which detects the hydraulic pressure of an oil chamber on one side is provided, and a second pressure detector 12 which detects the hydraulic pressure of an oil chamber on the other side is provided, The differential- pressure detecting device is constituted in such a way that the pressure detectors 11, 12 are provided respectively with magnetostrictive elements 14, 15 which receive the hydraulic pressure on one side in the axial direction that the pressure detectors are provided respectively with detecting coils 26, 27 by which magnetic fluxes advancing toward the axial-line direction are generated in the magnetostrictive elements 14, 15 and which generate a change in an inductance according to the magnetostrictive effect of the magnetostrictive elements 14, 15 and that the change in the inductance of both detecting coils 26, 27 is fetched as the voltage difference between both coils.