FO OF AN ACR 4 ENG WDB OVERSHOT DSCNT ALT DURING APCH RESULTING IN FO CORRECTING BACK AT ONCE AND ATC CALLED TO HAVE CREW CHK THEIR ALT.

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WE DEPARTED NARITA; JAPAN; AT XX30 NOV/XA/98 ENRTE TO ANC. IT'S AN ALL NIGHT FLT -- 5 HRS 50 MINS. WE WERE ASKED TO KEEP OUR SPD UP ON DSCNT TO ACCOMMODATE A FLT BEHIND US. THE FO WAS FLYING. BELOW 10000 FT WE WERE GIVEN VECTORS AWAY FROM THE DIRECT TRACK TO THE ARPT TO INCREASE SPACING WITH THE ACFT IN FRONT OF US. WE WERE CLRED TO 2000 FT AND HAD THE ACFT AHEAD OF US IN SIGHT; BUT THERE WAS LOW FOG IN THE AREA BELOW US. WE WERE THEN VECTORED TOWARD THE LOC. AT 4000 FT I SAID '2 TO GO' (2000 FT); AND AT 3000 FT I SAID '1 TO GO' (1000 FT). WE WERE THEN CLRED FOR THE APCH AND I SAID TO THE FO 'NOW WE CAN GO DOWN TO 1600 FT;' THE INITIAL APCH ALT TO RWY 6R AT ANC. THE SO AND I WERE THEN FOCUSED ON THE TFC AHEAD AND WHEN I LOOKED AT THE ALTIMETER WE WERE AT 1400 FT AND DSNDING. I SAID; WATCH YOUR ALT WE'RE TOO LOW. WE WERE ABOUT 12 MI W OF THE ARPT. THE FO CORRECTED BACK TO 1600 FT BUT IN THE PROCESS WE DSNDED TO ABOUT 1250 FT BEFORE THE DSCNT WAS ARRESTED. APCH CTL CALLED AND SAID TO CHK OUR ALT AS THE FO WAS ALREADY CORRECTING BACK. A NORMAL APCH AND LNDG WAS SUBSEQUENTLY ACCOMPLISHED. RVR WAS 2400 FT AND THE WIND WAS LIGHT AND VARIABLE. A HUMAN ERROR DUE TO INATTN AND BEING UP ALL NIGHT CONTRIBUTED TO THIS MISTAKE.