We have applied FT-IR remote sensing to the measurement of the volcanic gases of Aso volcano, central Kyushu, Japan, which has inaccessible fumaroles in an active crater. The observations were carried out three times between August 1996 and August 1997. As a result, in all observations, CO, CO
2, SO
2 and HCl were remotely detected and those column concentrations were estimated by comparison between theoretical and observed spectra. The observed CO/CO
2 and CO
2/SO
2 changed accompanying with the fumarolic temperamre. The observed CO
2/SO
2 decreased from August 1996 to April 1997 and increased from April 1997 to August 1997. As the variation of CO
2/SO
2 may attribute to the depth of magma degassing, the depth of magma degassing may have become shallow from August 1996 to April 1997 and deep from April 1997 to August 1997. The observed HCl/SO
2 showed low values (0.02 to 0.04) during the period. They are typical values at the quiescent stage of this volcano.
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