Knowing a child's affiliative feelings about a peer helps us understand child's social behavior toward peers and can predict how a relationship between two children would continue. A picture-drawing task, in which a child draws himself or herself and a peer, is a potentially valid way to measure a child's feelings of affiliation toward the peer. In this study, we established the validity of the picture-drawing task by testing two hypotheses: whether a child's higher affiliation toward a peer would relate to a shorter distance between the drawn child and the drawn peer (Hypothesis a) and whether the child's temporal variation of affiliation would relate to a change in the distance between two drawn figures across two time points (Hypothesis b). Forty-five children aged 3 to 6 attending the same nursery school in Japan drew pictures of themselves and a schoolmate. To determine who would draw whom in the picture-drawing task, we conducted a friend-nomination task and used social network analyses with such data to compute each child's degree of affiliation toward every schoolmate. The analyses supported both hypotheses: the distance in the drawing was shorter when the children drew a high-affiliation peer than a low-affiliation peer, and the distance in the picture became longer as the child's affiliation toward a peer decreased over time. Our results strengthen the validity of the picture-drawing task for measuring children's affiliation toward a peer.