An electromagnetic vibrating diaphragm pump capable of preventing fluid such as air from leaking to an oscillator side. This diaphragm is sandwiched by an inner center plate and an outer center plate and is fixed by inserting the mounting screw parts of the oscillator into a through hole provided at the center of the outer and inner center plates. A cylindrical projecting portion is formed at the center of the inner center plate on its oscillator side, and this projecting portion is fitted into the concave groove formed at the ends of the supporting member of the oscillator so as to be sealed in an airtight manner with a ring-shaped elastic member inbetween.