Based on the success of the Japanese asteroid explorer Hayabusa, the ECR ion thruster μ10 will be installed in Hayabusa's successor, Hayabusa-2, and is scheduled to be commercialized for use in geostationary satellites within the next three years. To increase the thrust force of the μ10 as much as possible without major design changes, luminescence measurements were conducted using an optical fiber probe. The probe gave an internal view of the μ10, and it was discovered that there was plasma in the waveguide. As the plasma, the density of which is higher than the cut-off density, interferes with the transmittance of microwaves, the propellant injection location was changed. In addition to the change in propellant injection location, the grid system was also refined. These improvements increased the thrust force from 8.0 mN to 10.1 mN with a decrease in specific impulse by 40 sec from 3200 sec to 3160 sec.