[horde] pear.php error...

[Seunghyun. Cho]

I've installed horde & horde-imp on my gentoo box.
When I open my imp page, I've got this error.

Warning: main(PEAR.php): failed to open stream: No such file or 
directory in /var/www/localhost/htdocs/horde/lib/Registry.php on line 13

Fatal error: main(): Failed opening required 'PEAR.php' 
(include_path='.:') in /var/www/localhost/htdocs/horde/lib/Registry.php 
on line 13

I couldn't file pear.php anywhere.
What should I do?