This paper proposes some new rules of rhythm in Japanese, based on the Center of Energy Gravity of Vowels: CEGV. These rules have risen from isochrony of mora, which linguists consider a characteristic feature of Japanese. Assuming the CEGV as the timing point of rhythm, and the duration between CEGV's(DceflV) as the parameter to determine the rhythm, we can form the Japanese rhythm simply by using only 29 rules (the first approximate rule-set). The isochrony of more than two moras is said to be another Japanese feature. Based on this, an additional rhythm rule (the second approximate i%ule) is proposed to prevent rhythm disturbances due to some exceptional combinations of the rules in the first approximate rule-set.