Class 'Memcache' not found

FILE: /www/wwwroot/creater.620309.com/ThinkPHP/Library/Think/Session/Driver/Memcache.class.php 　LINE: 22

#0 [internal function]: Think\Session\Driver\Memcache->open()
#1 /www/wwwroot/creater.620309.com/ThinkPHP/Common/functions.php(1319): session_start()
#2 /www/wwwroot/creater.620309.com/ThinkPHP/Library/Think/App.class.php(200): session()
#3 /www/wwwroot/creater.620309.com/ThinkPHP/Library/Think/Think.class.php(120): Think\App::run()
#4 /www/wwwroot/creater.620309.com/ThinkPHP/ThinkPHP.php(97): Think\Think::start()
#5 /www/wwwroot/creater.620309.com/index.php(9): require('/www/wwwroot/cr...')
#6 {main}