Knowledge share communities such as Q&A communities have been expanding in recent years. However, it might be difficult to understand how participant interactions build such a community site while their motivation to participate is clear. We focus on users' motivation (especially, reciprocity) on pecuniary transaction. In this paper, we model a user's interaction rule in a knowledge share community site, simulate our model, and discuss on the key factor for the growth of the community. According to the result of our simulation, we investigate the mechanism of that the degree of pecuniary for transactions of Q&As has much effect on the quantity and quality of answers for users' questions.