运行rails服务器时出错

/usr/bin/ruby1.9.1 extconf.rb
/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in 

`require': cannot load such file -- mkmf (LoadError)
    from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
    from extconf.rb:1:in `<main>'


Gem files will remain installed in /var/lib/gems/1.9.1/gems/json-1.8.6 for inspection.
Results logged to /var/lib/gems/1.9.1/gems/json-1.8.6/ext/json/ext/generator/gem_make.out

An error occurred while installing json (1.8.6), and Bundler cannot continue.
Make sure that `gem install json -v '1.8.6'` succeeds before bundling.
/usr/bin/ruby1.9.1 extconf.rb
/usr/lib/ruby/1.9