(1)First, repeated ultimate game was carried out using a computer simulation. As a result, we confirmed that we never behave rationally as standard economics assumes. (2)Second, on the basis of prospect theory, the risk aversion and seeking tendencies were analyzed in gambling situations. It tended that the participant behaved risk-aversively in a gain condition, and showed a risk-seeking behavior in a loss condition. (3)Third, on the basis of the loss aversion property derived from prospect theory, we discussed the endowment effect. Stronger endowment effect was observed when the affection was attached to the object than when the affection was not attached to the object at all. (4)Fourth, as a result of exploring the relationship between the diversification of risk (accepting (taking) the total risk gradually by dividing it into a few steps) and the risk assessment, we tended to underestimate the total risk when the total risk was divided into a few risk-taking behaviors than when the total risk was taken at a time.