"I have done my program of decoding AT keyboard scancode. Basicly it prints out scancodes whatever key I pressed."

When you say you have "done" this program, I assume that means you have tested it with a real AT keyboard and seen it working?

"I think the scanner protocol is the same as AT keyboard protocol, since the scanner use 'keyboard wedge'"

Are you sure that this scanner works at all?
Have you tried it with a PC?

You should be able to monitor the interface to see if the PC has to send any special codes to the scanner to get it working.
Your 8051 would then have to do the same.

If you are a bona fide developer, you could try contacting the scanner manufacturer?