I can't install appdmg!!

[icarusion]

I try many solutions, but still can not install appdmg in my Mac. Error info is:

npm ERR! Darwin 16.5.0
npm ERR! argv "/usr/local/bin/node" "/usr/local/bin/npm" "install" "appdmg" "-g"
npm ERR! node v7.10.0
npm ERR! npm  v4.2.0
npm ERR! path /usr/local/lib/node_modules
npm ERR! code EACCES
npm ERR! errno -13
npm ERR! syscall access

npm ERR! Error: EACCES: permission denied, access '/usr/local/lib/node_modules'
npm ERR!  { Error: EACCES: permission denied, access '/usr/local/lib/node_modules'
npm ERR!   errno: -13,
npm ERR!   code: 'EACCES',
npm ERR!   syscall: 'access',
npm ERR!   path: '/usr/local/lib/node_modules' }
npm ERR! 
npm ERR! Please try running this command again as root/Administrator.

Please help me, thank you!

[icarusion]

using sudo it worked