Aerosol optical thickness (AOT) was measured during Antarctic research cruises of R V Umitaka-Maru. The measured AOTs at 500 nm in the Indian sector of the Antarctic Ocean ranged from 0.02 to 0.12. In Antarctic coastal waters, the AOTs were as low as around 0.05. On the other hand, the some AOTs were higher than 0.1 from 40゜S to 60゜S. In middle and low latitude regions of the Indian Ocean, the AOTs ranged from 0.05 to 0.15. The AOTs were correlated with wind speed. It is indicated that sea-salt particles, generated by wind, dominated optically and wind speed affected the AOT in the Antarctic Ocean. Angstro¨m exponent decreased with increasing AOT. In the AOT wavelength spectra, some AOTs in 380 nm and 870 nm tended to deviate from the power function. Comparing calculation methods of Rayleigh optical thickness suggested a possibility of reducing the deviations of these AOTs. In the future, accuracy check of the instrument constant and review of the calculation method of Rayleigh optical thickness are recommended to enhance the accuracy of maritime AOT measurement.