I went on Avex's site and the mailing address I've found was this:
AVEX INC.
Head quartered 3-1-30, Minami-Aoyama, Minato-ku, Tokyo 107-8577 [Map]
TEL 03-5413-8550 (reception desk)
Established April 11, 1988
Capital 4,229.6 million yen
Representative Chairman / Chief Executive Officer
Tatsumi " Tom" Yoda
No. of employees 359 (avex group: 804)
http://www.avex.co.jp/e_site/index.html
Hope this helps!