PURPOSE: To contrive phase interference without joining both fluids by composing a pulsation reducing device of an interference device for giving the phase interference to fluid introduced in two introducing paths through an elastic diaphragm and a supply path for taking out individually fluid subjected to the phase interference by the elastic diaphragm.

CONSTITUTION: Both fluid introduced from both introducing paths 31, 32 are introduced into interference paths 11, 12 without uniting (joining) with each other and act on each other (mutual interference) through an elastic diaphragm 19. Then, since both fluid are introduced with the phase difference therebetween, the elastic diaphragm 19 is displaced according to the differential pressure between both fluid, so that energy is converted relatively between both fluid. Thus, fluid having mutual pulsating phenomenon reduced by the phase interference is supplied to supply paths 51, 52 provided from the interference path 11, 12.