Unable to start rails server + rails 4 + ruby-2.0.0-p0

When I run the rails server

$ rails server

Then I am getting below error

/home/rails/.rvm/rubies/ruby-2.0.0-p0/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require': cannot load such file -- active_support (LoadError)
from /home/rails/.rvm/rubies/ruby-2.0.0-p0/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require'
from /usr/lib/ruby/vendor_ruby/railties/lib/rails_generator.rb:30:in `<top (required)>'
from /home/rails/.rvm/rubies/ruby-2.0.0-p0/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require'
from /home/rails/.rvm/rubies/ruby-2.0.0-p0/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require'
from /usr/lib/ruby/vendor_ruby/railties/bin/rails:16:in `<main>'

Could you please help me and let me know why I am getting this error.