In a reciprocating pump comprising a reciprocating member adapted to reciprocate as driven by a drive unit contained in a drive unit case, so as to perform a pumping action such that a liquid in use is aspirated into a pump chamber formed axially in front of the reciprocating member and pushed out under pressure and further comprising a high-pressure seal in slidable contact with an outer peripheral surface of the reciprocating member axially behind the pump chamber so as to prevent the liquid in use from leaking out when a high pressure is generated in the pump chamber, a flow path for the liquid in use to flow so as to bathe the drive unit therewith is provided within the drive unit case, while the rear side of the high-pressure seal is in contact with the liquid in use flowing through the drive unit case.