rubygem problem

[Wang Liang]

Hi,all
I installed ruby from the source list,but when I run "gem",it gets this:
/usr/bin/gem:3:in `require': No such file to load -- rubygems (LoadError)
        from /usr/bin/gem:3
my Ubuntu: Ubuntu Linux 5.04: The Hoary Hedgehog Release
Anyboy can help me?