A 2.45 GHz compact National Institute of Radiological Sciences electron cyclotron resonance (NIRS-ECR) ion source has been developed for heavy ion medical accelerator in Chiba at NIRS. A new ECR source is expected to produceC2+ions of more than 160 e&mgr;A for the high-energy heavy-ion cancer treatment. An ECR-type ion source is adopted because of its excellent characteristics of long lifetime and easy operation. The compact ECR ion source is 15 cm in diameter, 20 cm in length, and about 20 kg in weight. A set of permanent magnets was adopted to generate both the axial mirror and the radial sextupole fields. Several gas materials, He,CO2,CH4,N2,and Ne, have been tested. The present performance forC2+ions, however, is 15 e&mgr;A and far below the medical requirements. The estimated vacuum in the plasma chamber is around5.0×10−5 Torr.A much better vacuum pressure is desired to produceC2+ions of more than 100 e&mgr;A. The ECR plasma, however, cannot be kept stable under such a high vacuum pressure. As another method to get the stable plasma, the different microwave injection was tested with a microwave antenna. ©1998 American Institute of Physics.