Sony Ericsson will offer in Japan a walkman cellphone next month that will enable users to download music onto their handsets in a partnership with No. 2 mobile carrier KDDI.

In Japan, more then 90% of the music downloads are on the phones, not on the PC, so such a product is a must for the Japanese market.

Japan boasts one of the world’s most sophisticated mobile phone markets, with millions of people using handsets to exchange e-mail, do Net restaurant searches, watch digital TV and play video games.

The handsets from Sony Ericsson will come with 1 gigabyte of built-in memory that’s enough to store 630 songs, and will play for 30 hours straight, company officials said.

Since August next year, Sony sold 5.5 million Walkman phones in the rest of the world, including the United States.

Unlike those sold elsewhere, the Walkman phones in Japan will connect to a digital music store operated by KDDI called Listen Mobile Service, or Lismo, for downloading music and will also download tunes directly into the cellphone in a separate KDDI service called “chaku-uta” that’s already popular in Japan.

KDDI leads the Japanese market in cellphones that download music, and 47 million songs have already been downloaded through the chaku-uta service. But in the mobile phone market, it trails Japan’s top mobile carrier NTT DoCoMo, which has about a 60% market share.

Koji Otsuka, a KDDI official, said the Walkman phone was an important part of the company’s defenses ahead of number portability.

“We want to offer a wide variety of products through our powerful brand collaboration with Sony,” he said. “We are a brand that caters to music lovers.”

That’s all from Japan.