Heliborne measurements of CO2/SO2 ratio in a volcanic plume were performed at Miyakejima volcano, which has degassed intensively since September, 2000. The repeated measurements started in October, 2000 and showed constant molar CO2/SO2 ratio at 0.75 ¿ 0.25 despite decreased SO2 emission rate from 42 kt/d in late 2000 to 16 kt/d in late 2001. The CO2 emission rate also decreased from 21 kt/d in late 2000 to 8 kt/d in late 2001. The CO2 content in the magma is estimated to be 1200 ppm based on the mole ratio and S content in melt inclusions (1300 ppm). The estimated CO2 content corresponds to a saturation pressure of 280 MPa suggesting the presence of CO2-rich bubbles in the magma chamber located at 4 km. The constant CO2/SO2 ratio over the year implies a stable magma degassing process without a change in the composition of the degassing magma.