This essay clarifies the context of yojo (personal health care) from the mid 16th Century through the mid 17th Century. Yojo was a field of medicine and was not popular in the 16th Century. Only medical practitioners and Army Generals who had learned yojo from the former had the knowledge of yojo. However, after the 18th Century yojo became popular and a kind of ethics. This situation was caused by a change in the perception of the value of life due to the ending of the war era and the new Togugawa era of peace.