An ion generation apparatus utilizes microwaves and employs the electron cyclotron resonance phenomenon to generate plasma. The plasma is confined in a plasma generation chamber by a mirror field, whereby high density plasma is obtained. A target disposed within the plasma generation chamber is sputtered by the ions in the high density plasma, so that a large number of ions is produced. This ion generation apparatus can be employed in a thin film forming apparatus which forms a thin film on the surface of a substrate by directing the ions and neutral particles to the substrate. An ion extracting grid may be included. Permanent magnets may be disposed at the upper and lower ends of the target disposed in the plasma generation chamber so as to permit the leakage of magnetic flux to the inner surface of the target. This permits the film to be formed at a high rate even when the voltage applied to the target is relatively low.