The comfort of Japanese style mattresses was examined in this study. Body movement was evaluated during sleep by an actigragh which was attached to the wrist of the subjects. The device can detect body movement acceleration of more than 0.01G (gravity acceleration). Japanese style mattresses were selected from the standpoint of their softness or hardness. Three types of Japanese style mattresses were used for the experiments, all of which were packed with urethane foam. Three subjects volunteered for the experiments, which were duplicated seven times on each mattress by each subject. In total, 63 experiments were conducted. Physical characteristics of the mattresses were measured in terms of their depression under the weight of the subjects. The activity index was computed by actigraph software to show the subjects' movement during sleep. As a result, the least body movement was observed when the soft and thick Japanese style mattress was used. The second least movement was observed when the hard and thin mattress was used, followed by the hard and thick mattress.