Hi Kenichi Watanabe:
Would seem some Apple reference reported the Magic Mouse batteries should last four months. I've only found items that report the reference (have not located the reference itself). Clips below are cross posted, but if you search the Internet for the terms ["Magic Mouse" four months] (without the brackets), you'll find a whole series of early reviews and comments containing the same reference to four month battery life.
Perhaps inquire directly of Apple Care. Gizmodo's review (
http://gizmodo.com/5386202/apple-magic-mouse-review ) relays a claim about the Magic Mouse battery life saying, "it's powered by two AA batteries, which get up to four months of use per charge, according to Apple."
Engadget's preview (link follows) included the same reference, the related headline read, "Apple's Magic Mouse: one button, multitouch gestures, Bluetooth, four-month battery life."
http://www.engadget.com/2009/10/20/apples-magic-mouse-multitouch-gestures-blueto oth-four-month/
Finding support for Gizmodo's reference ("according to Apple") and Engadget's headline may be a little more difficult. Perhaps I haven't read the press release carefully enough. Clip below, from that Apple press release:
"Apple Introduces Magic Mouse," press release, 20 Oct 2009, Apple.com (
http://www.apple.com/pr/library/2009/10/20magicmouse.html : accessed 24 Nov 2009), in part, "Magic Mouse uses Bluetooth wireless capabilities to create a clean, cable-free desk top and its secure wireless connection works from up to 10 meters away. To extend battery performance, Magic Mouse includes an advanced power management system that works with Mac OS® X to automatically switch to low power modes during periods of inactivity. The wireless Magic Mouse is powered by two AA batteries which are included."
Hope this helps