And I'm going to try to downgrade it, but I'm afraid it's right.
1. Use ASUS Flasher with HL-DT-ST-BD-RE_BU40N-BU12-OM01001-211902230922.bin(BU40N-B-Japan-Buffalo folder).
(Or with HL-DT-ST-BD-RE_BU40N-1.04-NM00500-212005061142(BU40N-N-Main folder)? Confused..)
2. Flash it again with DE_Buffalo_BRUHD-PU3_BU10.
* At first, I tried to downgrade it without MK(only with DE_Buffalo_BRUHD-PU3_BU10), but failure #4 popped up.

Please make sure it's correct.