The aim of the present study is to reveal what environmental composition (things and people which help children) and assistance is needed for three-year-old children to accumulate experience of feeling, thinking, and trial to foster "the development of thinking ability". In order to reveal them, this study employed teachers' conferences based on practical cases which three-year-old children feel, think, and try something when they got involved with others and things around them. The result of the study shows that the following environment and assistance to make children foster thinking ability are needed: (l) teachers' assistance to think together and sympathize with the children when they have interest in natural events: (2) aids which accept their :findings and ask questions to deepen their thought: (3) teachers' aids to encourage the children to continue tackling what they want to do: (4) an environment in which the children are able to try and duplicate what they want to do : (5) environment and aids to expand involvement and opportunities to be exposed to the other children's' :findings and devices.