MD11 EXPERIENCED LTSS WITH A DEPARTING B747 WHILE LNDG AT NARITA; JAPAN.

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CONDITIONS: IMC; ILS RWY 34L NARITA; JAPAN; FO FLYING. CAPT; PNF; VREF 160 KTS; V-APCH 171 KTS DUE TO 11 KT WIND GUST; RELIEF FO IN JUMPSEAT. APPROX 10 NM OUT AND CLRED FOR APCH; APCH CTL ASKED OUR SPD. I SAID 170 KTS AND THAT'S OUR MINIMUM. SWITCHED TO TWR AND TWR SAID CONTINUE APCH AND SLOW TO MINIMUM APCH SPD. I ADVISED WE WERE SLOWED TO MINIMUM. TWR THEN CLRED ONE ACFT FOR TKOF AND TOLD A B747 TO TAXI INTO POS RWY 34L. WE WERE STILL IMC AND PROBABLY 5 MI OUT. AT ABOUT 1000 FT AGL; THE B747 WAS CLRED FOR TKOF BUT PROBABLY WAS NOT YET IN POS. AT 500 FT AGL; TWR ADVISED WE WOULD GET A LATE LNDG CLRNC. WE BROKE OUT OF THE CLOUDS AT 300 FT AGL TO SEE THE B747 SLOWLY ACCELERATING ABOUT 1/3 DOWN THE RWY. I ADVISED THE CREW THAT A GAR WOULD PROBABLY BE WORSE FOR SEPARATION THAN LNDG. WE CONTINUED THE APCH; GOT LNDG CLRNC BELOW 70 FT AGL; AND TOUCHED DOWN JUST AFTER THE B747 BROKE GND. I WOULD ESTIMATE SEPARATION AT LESS THAN 5000 FT. I HAD AN ANGRY EXCHANGE WITH THE TWR CTLR AFTER LNDG. THE TWR TOOK FULL RESPONSIBILITY. HAD WE GONE AROUND AND HAD AN ENG FAILURE OR COM PROB; WE WOULD HAVE FLOWN DIRECTLY OVER THE B747. HAD HE REJECTED THE TKOF; WE WOULD HAVE BEEN FORCED TO DO A LOW LEVEL GAR IN IMC CONDITIONS. SUPPLEMENTAL INFO FROM ACN 611863: OUR CREW WAS CONCERNED THAT IF WE HAD GONE MISSED APCH WE WOULD HAVE 'CAUGHT' DEPARTING B747 AND WOULD HAVE RETURNED TO IMC. LNDG FLARE AND ROLLOUT UNEVENTFUL EXCEPT THAT ENCOUNTERED SOME WAKE TURB FROM DEPARTING B747. MISSED APCH PROC WAS L TURN AT 600 FT. WE WOULD HAVE BEEN 'ON TOP' OF THE B747 AT 600 FT.