The fake cops stopped the group and demanded briefcases that contained the yellow metal, telling them: "We know it's smuggled," the Asahi Shimbun reported.
Apparently taken in by the disguise, the victims gave the pretend police what they asked for and watched as they drove off.
The heist, in Fukuoka, happened as the men were on their way to sell the more-than 100 kilogrammes (220 pounds) ofgold, said reports, which valued the haul at 600 million yen ($5.2 million).
The men who were robbed later told the real police they had bought the metal the day before.
An official police spokesman in Fukuoka refused to confirm the reports, which said the theft took place in July, and most likely involved organised crime gangs.
