To obtain a required volume of air at a low power consumption, and to easily adjust a natural frequency of an oscillation system even when a diaphragm tension is changed by variations of products and the like, by providing a balancer for adjusting the natural frequency of the oscillation system between a center plate and an oscillator.

In an electromagnetic diaphragm pump suitable for air-seasoning of clarifying tank and supplying oxygen to cultured fish, and deforming a diaphragm 40 arranged on both sides of an oscillator 30 by reciprocating the oscillator 30 according to a polarity change of an electromagnet 21 in order to intake and discharge a fluid body, a balancer 200 for adjusting a natural frequency of an oscillation system is provided between center plates 70, 80, formed in a middle part of the diaphragm 40 so as to clip the diaphragm 40, and the oscillator 30. Therefore, the tension of the diaphragm 40, the weight of the center plates 70, 80, and the weight of the oscillator 30 are adjusted, and fine adjustment of the natural frequency is enabled by the addition of the selected balancer 200.