In this paper, the author tried to investigate development of children's responses to rhythm by taking "synchronization" as a clue. The subjects used ranged from two years old to six. They required to respond to nine kinds of rhythmic patterns by handedness. The results obtained are summarized as follows. 1. When children are about three years old, they reach a stage where they can respond to simple rhythmic pattern, and show elementary synchronization with rhythm. It is interpreted that rhythm for children change its function from as an energizer to organizer. 2. With increasing ages, infants become respond to rhythmic patterns more quickly and skilIfully. At about six years old children can respond to almost all patterns. 3. As wrong responses, following four types were found. (a) "Grouping" is correct, but responses do not match for the sound of stimuli.(b) [figure] is mistaken with "〓". (c) [figure] is mistaken with [figure], or [figure].(d) Random responses. 4. The patterns including pauses can not be easily synchronized. This is true throughout all the ages. But once whole pattern is grasped, wrong responses appear seldom. 5. The responses correctly responded to the accent of stimuli development gradually in three, four, and five years old, and suddenly in six.