It is known that the Ultimatum Game is an index of human social interaction behavior with regard to inequality aversion , correction of illegitimacy and egalitarianism. Previous research found that preference for fairness tends to occur with Theory of Mind. In general, children enhance their sociability through playing with peers. However, taking into account gender difference of playing style preference, it is possible to predict that social abilities differ by gender. In this research to find a gender difference of social behavior in playing with peers, new game based on the Ultimatum Game was constructed. Participants were 9 or 10 years old and played with 3 other same gender peers (one is a friend and two are foes) according to some rules. As a result, male tends to value his friend in comparison with female. Predictable gender difference of social abilities caused by observed behavioral traits was discussed.