In our modern society trust is very important as the moral infrastructure of society. To become trustworthy person, we must be sincere. Where does this sincerity appear? For example, in the relation between doctor and patient or between salesclerk and shopper, we usually require the sincerity only from doctor and salesclerk. Namely sincerity is asked against the side which has the duty to respond. Then sincerity (Seijitsu in Japanese) and loyalty (Chuujitsu, Chuusei, Chuugi in Japanese) as the exertion with devoted mind,have resemblance, but their difference is great. The latter all devote entirely their mind and body and are dependent on their lord. But the former sincerity has the devotion only in the scene of duty and stands in equal independent relation to the claimable person. Sincerity (Seijitsu) is the virtue of modern democratic society in opposition to the feudal loyalty (Chuugi). In our society social trust is very important for the quite certain exchange or social relations. Fundamentally for this trust we must have the technical high ability to be requested. And secondly it is important to hold the sincerity. Trust and sincerity can form the good social cycle in our modern society. Since the duty is not cheerful, but usually disgusting matter, the sincerity as devotion cannot be formed naturally. Sincerity as higher behavior or noble virtue is in the duty rarely formed. But in the field of law this sincerity is as ""good faith"" not noble virtue but only to fulfill duty or obligation. The sincerity as ""good faith"" is not virtue but mere legality to perform a duty. Therefore we have two type of sincerity. One is the noble high virtue in response to righteous request and the other is the fundamental duty in response to lawful right.