A reciprocating fluid transfer pump in which a pilot valve assembly block can be mounted to a main body block in a simple, easy and reliable manner. In the reciprocating fluid transfer pump, a fluid, which has been suctioned into a fluid delivering chamber (19), is discharged therefrom by causing an expanding and contracting action of a barrier membrane (6) which is attached to either end of a center rod (5). The center rod is capable of reciprocating with respect to the main body block (2). A peripheral portion of the barrier membrane (6) is clamped fixedly to the main body block (2) via a main ring (30), and a pilot valve assembly block (17) having an integrated pilot valve (16) for detecting the reciprocating motion of the center rod (5) is pressed against and thus fixedly secured to the main ring (30) via a sub ring (34).