The present invention provides a mass spectrometer including an ion source for atomizing a liquid sample into ionized droplets and spraying ions in a predetermined direction. According to the present invention, the ion source includes a gas transport pipe and a liquid supply pipe; the gas transport pipe has an ejection port at its front end and a gas supply passage for sending an assist gas to the ejection port; the inner surface of the gas supply passage has a tapered section located in proximity to the ejection port, where the diameter of the tapered section decreases toward the ejection port; the liquid supply pipe is inserted into the gas supply passage so that the front end of the liquid supply pipe is located in proximity to the ejection port; three or more spheres having the same size are inserted between the inner surface of the gas supply passage and the outer surface of the liquid supply pipe; and a pressing mechanism is used to press the spheres onto the tapered section. Being pressed by the pressing mechanism, the spheres move along the tapered section and come closer to the central axis of the liquid supply passage. The gas transport pipe and the liquid supply pipe form a duplex pipe structure having a high degree of coaxiality, which produces a stable flow of ions sprayed in the predetermined direction.