Concentrations of chlorine and sulfur bearing species in volcanic gases from the Sakurajima Volcano have been continuously measured by a variety of collection techniques and analytical methods. The results of analyses on water-soluble components in volcanic ashes, gases absorbed into alkaline solutions, and gases diffused into the atmosphere revealed that HCl/SO₂ ratio increased prior to any surface activity such as frequency of explosions. It is suggested that HCl/SO₂ ratio can be one of the useful precursory indicators for volcanic activity.