Onomatopoeia expression is a sensitive word and an important language element of Japanese. But research on it, especially during preschool children, has not yet been sufficiently developed. This research investigates the function of preschool children's onomatopoeia expression, which is based on observations on their activities in a natural setting. The first purpose of this report is to mention two characteristics of onomatopoeia expression, namely expression variety and lexical ambiguity. The second purpose is to show the effectiveness of onomatopoeia expression in estimating their development of cognitive notice. The results of this case study indicate that: 1) Onomatopoeia expression greatly depends on the context or situation. 2) Onomatopoeia expression may become the key to understanding preschool children's development of cognitive notice. These two points will continue to be studied in future investigation.