Thanks everyone for your advice for us!!
However, I'm confusing by some opinions about this.
Unfortunately, we are fixed the hardware board based on atmega2560 & 16u2.
We cannot use CH340 or other serial-USB device.
1) Mr. avr_fred said "OK, but you must include a copy of the license agreement with your distribution."
We supposed that it's Ok as you said. Because, the comment is including in arduino.inf.
; Arduino is free software; you can redistribute it and/or modify
; it under the terms of the GNU General Public License as published by
; the Free Software Foundation; either version 2 of the License, or
; (at your option) any later version.
; This program is distributed in the hope that it will be useful,
; but WITHOUT ANY WARRANTY; without even the implied warranty of
; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
; GNU General Public License for more details.
skip the rest...
2) Mr. westfw said "Depending on what "based on" means, you perhaps are supposed to get your own USB VID (Vendor ID) and create your own .inf and .cat files. It's pretty much a no-no to have your non-arduino device identify itself as an "Arduino Mega from Arduino Inc" (based on USB Implementers Forum rules, not source-code issues.) (It would be OK if you were actually including an Arduino Mega BOARD inside your product.)
Our product is a original board based on Mega2560 & 16u2, but not pure Arduino Mega BOARD.
We considered about vid/pid as you said.
However, we thought we can redistribute the driver as Mr.avr_fred & arduino.inf's comment.
I want to ask you suggestion again as against above our supposition & Mr.avr_fred's suggestion.
If we cannot redistribute that driver, we thought distribute our product attached with our original modified partially free software(GPL) package which including that driver (arduino.cat & arduino.inf).