Django version is 1.4. I had read the official document, and googled my problem.
first I had followed the official document Managing static files added
Unfortunately, Django's render_to_response shortcut by default uses normal template context, which does not include context processors and all their fancy and useful stuff like STATIC_URL. You need to use RequestContext, which does precicely that.
This can be called by using the new render (available since Django 1.3):
from django.shortcuts import render
return render(request, 'register.html', {'errors':errors})
In Django 1.2 and older, you need to supply the context explicitly:
from django.shortcuts import render_to_response
from django.template import RequestContext
return render_to_response('register.html', {'errors':errors},
context_instance=RequestContext(request))