An infusion pump comprises an elastomeric hemispheric deformable rolling diaphragm, an external rigid housing with internal area no greater than that of the rolling diaphragm exterior, and a rigid cap structure which sealingly fastens the rim of the rolling diaphragm to the rim of the external housing. Thickness of the flexing wall of the diaphragm is tapered from rim to a region on the central axis to provide approximately constant pressure as the pump empties its contents. The rigid housing has an port in its center opposite its rim to allow insertion of fluid to roll the diaphragm inward, and to allow outflow of the stored pressurized fluid as desired. The housing and/or cap structure may have an external belt mounting fixture to facilitate donning the infusion pump for ambulatory users. The simplified design enables low manufacturing cost and hence single-use disposability.