In most Japanese preschools, children are involved in setting up lunches as monitors' activities. The current study conducted six-month long naturalistic observations of lunch monitors' activities in a nursery school in Tokyo. Ten five-year-old children were included in the analysis. By engaging in the activities with their peers, the lunch monitors learned serving skills and how to serve food and related items accurately. There were individual differences in the type of cue children relied on while placing items, based on which they were divided into two groups: self-helping and social groups. Between these two groups, role-sharing and cooperative commitment were observed. The obtained results were discussed in terms of development of peer cooperation, the effect of small group activities with same members, and teachers' careful arrangements of small group activities.