unable to install json gem that seems a requirement

[sinsunsan]

ruby backup.rb ../
Faraday: you may want to install system_timer for reliable timeouts
/Library/Ruby/Site/1.8/rubygems/custom_require.rb:55:in gem_original_require': no such file to load -- json (LoadError) from /Library/Ruby/Site/1.8/rubygems/custom_require.rb:55:inrequire'
from backup.rb:4

Then I've tried to install json gem (I'm on a mac)

sudo gem install json
Fetching: json-1.8.0.gem (100%)
Building native extensions. This could take a while...
ERROR: Error installing json:
ERROR: Failed to build gem native extension.

    /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby extconf.rb

mkmf.rb can't find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/ruby.h

Gem files will remain installed in /Library/Ruby/Gems/1.8/gems/json-1.8.0 for inspection.
Results logged to /Library/Ruby/Gems/1.8/gems/json-1.8.0/ext/json/ext/generator/gem_make.out