The content of lithium in seaweed has been investigated for estimating the dietary intake of lithium, which has been known as a remedy medicine of manic-depressive psychosis. In this work, 46 samples of seaweed were divided into 5 groups; Hijiki 13, Wakame 15, Konbu 12, Arame 4 and Mekabu 4 samples. Lithium content was determined by flame photometry after the decomposition of a seaweed sample (∼1 g) with the mixture of nitric and perchloric acids. It was found that lithium content was obviously higher in “Hijiki” (3400±626 ng/g, n=13) than in other seaweed samples; “Wakame” (2000±1020 ng/g, n=15) and “Konbu” (1210±492 ng/g, n=12). It was indicated that lithium was present in seaweed “Hijiki” as insoluble salt associated with calcium or calcium complexes from the results of both lithium and calcium in chemical leaching experiments using water, 0.1 mol/l HCl, 6 mol/l HCl, 0.1 mol/l CH3COOH, 1 mol/l CH3COOH and 0.01 mol/l EDTA as a leaching solution.