A suction passage (1) and a discharge passage (2) for a to-be-transferred
fluid are disposed in a pump body (3). A bottomed cylindrical bellows (6)
is integrally coupled to the axial rear side of the pump body to form a
closed space (7). The suction passage (1) has an upstream portion (1A)
having a larger diameter and a predetermined passage cross section area,
and downstream portions (1B) which are formed by branching the upstream
portion into a bifurcated or Y-shape while reducing the passage cross
section area. Two first small check valves (15) of the spring type in
each of which the pressure receiving area is reduced in accordance with
the reduced passage cross section areas of the smaller-diameter
downstream portions (1B) are attached respectively to outlet portions of
the smaller-diameter down-stream portions (1B) so as to be arranged in
parallel. The outlets of the first check valves (15) are opened in the
closed space (7).