An ion source includes a body having a gas passage and an orifice. A capillary is inserted into the gas passage so that a tip portion of the capillary extends into the orifice. A gas supplier supplies a gas into the gas passage to form a gas flow through the gas passage along the capillary and through the orifice past a tip of the capillary so that the gas flow sprays a sample solution flowing through the capillary from the tip of the capillary. A flow controller regulates a pressure of the gas in the gas passage to adjust a characteristic value F/S to a predetermined value, where F is a flow rate of the gas flow at standard conditions (20.degree. C., 1 atmosphere), and S is a difference between a cross section of the orifice and a cross section of the tip portion of the capillary in the orifice.