PURPOSE: To directly introduce gas to be ionized to a portion high in ionization efficiency by providing a plurality of minute port for jetting gas to be ionized at the tip end section of a gas introduction pipe which is projected so as to be set in an ionization chamber.

CONSTITUTION: When gas 3aB to be ionized is fed to an ionization chamber 1 which undergoes an exciting condition by an ionization means 8, out of minute ports 3aA at the tip 3a of a gas introduction pipe 3 in a projected shape, gas 3aB to be ionized can be fed to a position highest in ionization efficiency within the ionization chamber 1 in an electrically insulated condition. Moreover, since the gas introduction pipe 3 and its tip 3a are formed with insulating material, potential distribution within the ionization chamber 1 will never be disturbed, so that the tip 3a can thereby be positioned at an objective place high in ionization efficiency within the chamber 1.