The present invention relates to a resin-made spring which, although the spring is made of a resin only, has a strength similar to that of a metal-made spring, and in which the strength is hardly reduced with time and can be maintained to a desired degree for a long term, and also to a bellows type constant volume pump which uses such a resin-made spring. The resin-made spring of the present invention has a configuration in which a strip-like PTFE-made sheet is bent in a zigzag manner in a longitudinal direction of the sheet to thereby provide a predetermined swelling curvature R, side faces of bent portions of the predetermined curvature are in close proximity to or in contact with each other, and the bent portions of the predetermined curvature are elastically expandable and contractible by means of heat holding and cooling. Such a PTFE-made spring is used as a spring for a check valve on the side of a tip end portion of a discharge hole of a bellows type constant volume pump.