Japanese wireless operator NTT DoCoMo is set to introduce a flat-rate mobile music service this coming May, according to reports.

The Japanese financial daily Nikkei writes that the operator will let subscribers download an unlimited number of songs per month for a fixed fee of about $17 a month. However the catalog of available tracks to download will be smaller than for other services that charge per song. The exact size of the catalog was not revealed.

The flat rate service will only be available on handsets that contain special software, which won't go on sale until May. It's unclear at this time whether this software is similar to that of subscription music services -- which lock out songs if subscribers stop paying their monthly fee.

NTT DoCoMo was an early star in the mobile Internet and entertainment market with its hugely popular imode service. However in recent years, rival KDDI has overtaken the company on the strength of its mobile music service -- Chaku Uta Full. KDDI was the first wireless operator to sell full-length tracks on an a la carte model.