Some 20 million songs were downloaded by KDDI's cellphone customers in six months to September, helping net profit to grow by 30 percent to 101.4 billion yen (880 million dollars) from a year earlier.
The rise came despite sluggish revenues which held steady at around 1.47 trillion yen, KDDI said in a statement.
The group's "Au" cellphone unit made a net profit of 111.3 billion yen, helping KDDI to absorb a 16.2-billion-yen loss on its fixed line business, which was partly the result of increase investment in a new optical network.
"Au", which counted 20.7 millions customers as of the end of September, has proved a hit among young customers, in part because of exclusive designs and online sales of music.