A mass spectrometric apparatus of high sensitivity, including a spray ionization interface suitable for the ionization of a low flow-rate liquid that prevents charged particles from being introduced into a vacuum device; wherein the ion source comprises a capillary having a first end having an inner diameter that gradually reduces in size in the direction of gas flow and wherein a liquid sample is introduced into an opposite second end of the capillary; a gas guide tube which guides gas flow along an outer periphery of the first end the capillary and which sprays the liquid sample from the first end of the capillary; and a gas introducing section for introducing the gas into the gas guide tube. A first end of the gas guide tube has a reduced inside diameter and receives the first end of the capillary in a holding member. Gaseous ions produced are introduced into a vacuum section through an ion intake port and are subjected to mass separation by a mass spectrometer. The angle between the central axis of the capillary and that of the ion intake port is greater than about 15.degree..