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Several research and developments have been performed at a compact ECR ion source (Kei3) with a fixed magnetic field at HIMAC. The Kei3 is a testing machine for application of Kei series developed for high-energy carbon-ion radiotherapy. It is difficult for Kei3 to produce ions that are heavier than carbon ion because it uses the same magnetic field distribution as the Kei series. In order to improve the beam current of multiply charged ions such as the neon and the argon, the two microwave frequency heating method was tested in Kei3. Microwaves are introduced from the vacuum box on the upstream side of the plasma chamber through the rectangular waveguide of the WR-90. A traveling wave tube (TWT) amplifier made by NEC (LD79X75A1) is used as the microwave source. The frequency band is 9.75-10.25 GHz and the maximum output power is 750 W. The WR-75 waveguide is also introduced from the upper part of the upstream vacuum box and installed on top of the existing WR-90 waveguide. A TWT amplifier manufactured by Xicom (XTRD-300IJ) was used as a microwave source for the two frequency heating. The frequency band is 10-18 GHz and the maximum output power is 300 W. A beam test with oxygen, neon, and argon was performed in order to confirm the effect of two microwave frequency heating. In this test, we focused on O6+, Ne7+, and Ar9+ ions. We checked the microwave frequency at the XTRD-300IJ, dependence of the microwave power at the LD79X75A1 and charge state distributions of O, Ne and Ar.