Previously, it has been suggested that mercury from the submarine volcanic gas which had been ejected from the lowest points of the Wakamiko caldera in the northern part of Kagoshima bay, is the source of methyl-mercury contamination of the bay, where no chemical industries using mercury are located. However, it was reported in another study that 3.2 tons of mercury from mercury pesticides used from the latter half of the 1950s to the 1970s had flowed into the bay. Environmental research which relates to mercury in the bay, reported by the Environment Agency of Kagoshima Prefecture and other researchers from the 1970s till now, was epidemiologically re-examined for the purpose of checking whether the northern bay had been polluted in the 1970s by mercury from submarine volcanic gas or from mercury pesticides. The samples of seawater (n =234) from each layer in the bay were collected on May, July, October and January from 1974 to 1975. The samples of seawater from the surface layer containing mercury higher than 0.5 ppb were especially high during July (statistical probability p<0.001). Moreover, the total mercury concentration of seawater in the bay from 1975 to 1983 became lower year by year (p<0.001), and that for three years from 1975 was high in July and August (p<0.001). The high concentration of mercury detected in the seawater from the surface and upper layers of the bay in summer during the latter half of the 1970s could not be removed because of the formation of a deep thermal cline in the bay in summer. On the other hand, in 1994, when there were small-scale eruptions in the area, the total mercury concentrations of the surface seawater collected in April, June, July, September, November and December were the lowest in July (p<0.001), but the highest in December (p<0.001). The author, therefore, believes that the source of mercury which had polluted the environment of Kagoshima bay in the summers of the 1970s was mercury-pesticides used for rice blast fungus treatment which had been carried by river water, but not the submarine volcano gas ejected at the bottom areas of the caldera.