a problem with mongrel install

[富丽饼干]

hi all:
when i install mongrel

Building native extensions. This could take a while...
extconf.rb:1:in `require': no such file to load -- mkmf (LoadError)
from extconf.rb:1

ERROR: Error installing gem mongrel-0.3.13.3.gem[.gem]: ERROR: Failed
to build gem native extension.
Gem files will remain installed in
/usr/lib/ruby/gems/1.8/gems/mongrel-0.3.13.3 for inspection.