In the present invention, palladium isotopes on the ground level are excited to the first excitation level using a first laser beam having a first wavelength. Palladium isotopes on the first excitation level are excited to the second excitation level using a second laser beam having a second wavelength. Palladium isotopes having an odd mass number are selectively excited to the second excitation level through the first and second excitation steps, and the identity of the ion-core state of the palladium isotopes is retained between the first excitation level and the second excitation level. Thefirst and second wavelengths are selected so that the second excitation level will be an auto-ionization level. Alternatively, when the second excitation level is not the auto-ionization level, a third laser beam that has a third wavelength is used in a third excitation step, and the first, second, and third wavelengths are selected so that the palladium isotopes at the second excitation level are excited to the auto-ionization level.