From the records in ruins of ancient Japan, everyone knows Japanese people have been eating seaweed since very early times in history, and it is an important foodstuff for Japanese cuisine. In recent years, due to changes in dietary habits of Japanese people, the incidence of lifestyle-related diseases such as cancer, diabetes, and Alzheimer's disease (AD), has increased. It was assumed that eating seaweed and its processed products could prevent such diseases. In order to verify this assumption, a series of animal experiments were conducted. Several types of seaweed proved to reduce the risk of developing colorectal cancer and breast cancer. Among them, Amanori (Porphyra yezoensis) was found to suppress the carcinogenic rate in intestine and breast remarkably. It was also found that impaired glucose tolerance was improved in Wakame (Undaria pinnatifida) diet with mouse model of type 2 diabetes. Furthermore, in the experiments with transgenic mouse model of AD, these results showed that eating of Wakame can be expected to prevent the effect of onset. In this way, it turned out that seaweed proved to effect on lifestyle-related diseases. On the other hand, seaweed contains iodine, so intermittent intake is recommended for overdose prevention.