The present invention provides a leakage-proof contrivance for upper hood of diaphragm pump by modifying the vertical wall of the lower tiered brim in the tiered cavity of the conventional pump upper hood into a sloped wall. Thereby, when stacked assembly of sealing collar gasket, piston valve and diaphragm is inset into the tiered cavity of the pump upper hood in closely attachment manner, the sealing collar gasket will be slantwise squeezed and vertically deformed to produce normal outwards stress in both upwards and downwards manners. Thus, the sealing collar gasket can respectively inset in the pump upper hood and piston valve in much more closely attachment manner to block and prevent the pressurized water from leaking out of the diaphragm pump.