PURPOSE: To obtain a high ionization efficiency with a small laser output, lower the density of the produced ions and reduce the loss of charge exchange.

CONSTITUTION: A metal material 3 is placed into an evaporating crucible 2 and irradiated with an electron beam 5 from an electron gun 4. The metal material 3 is heated and melted by the irradiation of the electron beam 5. The metal vapor stream 7 of the metal material 3 being evaporated flows through recovery electrode plates 8 toward a collecting recovery plate 10. Laser beam irradiation areas 9 are provided between the recovery electrode plates 8 in the same number as super-fine structure lines, becoming larger in the direction of the metal vapor stream 7. In each of the irradiation areas 9, the metal vapor stream 7 is irradiated with selective excitation laser beam to excite the metal atom of a specific isotope selectively by an optical resonance reaction.