Ion beam current extracted from a modified Bernas type ion source occasionally exhibits an oscillation at a frequency of several 100 kHz. Increase in the strength of a linear magnetic field induced to the ion source has either decreased or increased the frequency of this oscillation. The frequency showed an increase in proportion to the ion extraction voltage when the frequency decreased with increasing magnetic field. The change of extraction voltage did not affect the frequency when the frequency increased with the increasing magnetic field. Mechanisms causing these oscillations of an ion beam had been investigated.