Los Angeles-based digital music service Napster said today that the firm has launched its mobile service in Japan. Napster said that its mobile over-the-air song and ringtone service is now available on NTT DoCoMo's iMode mobile internet service, expanding Napster's service in the country into the mobile arena. Napster has been offering PC-based music subscription services in Japan last month with Tower Records Japan. The new service is available for approximately $3 per month.