In a mass spectrometer using a sonic spray ion source, a technique of controlling the density of droplets in a nebulized sample solution which is passed into a mass spectrometer at high vacuum to an appropriate value to thereby reduce analysis noises is disclosed. A sample solution in a sample solution injection unit 1 is introduced into a capillary 2 disposed in an ion source 6. A gas is introduced from a gas supply unit 4 by way of a gas pipe 5 into the ion source 6 and is caused to flow along the outer circumferential surface at the top end of the capillary 2 and is jetted out from the orifice 3 as a gas flow into atmospheric air. The sample solution jetted from the top end of the capillary 2 is ionized by the gas flow in the atmospheric air. Fine droplets or ions formed by the sonic spray method are collide against a diffuser 7, droplets and ions reduced for the density by the diffusion pass through the holes 8 disposed in the diffuser 7, and pass from the sample orifice 10 into a mass spectrometer 11 and mass analyzed. Provision of the diffuser 7 can suppress generation of analysis noises caused by the clustering phenomenon resulting from introduction of droplets or ions at high density into the mass spectrometer 11, thereby enabling to conduct analysis at high S/N ratio.