An unsuspecting passenger who flew through Tokyo's Narita airport left with $10,000 worth of free marijuana thanks to a forgetful customs officer and a sniffer dog with an unreliable nose.

The officer stuffed five ounces of the drug into the side pocket of a randomly selected black suitcase coming off an overseas flight into Narita yesterday so that the dog could get some practice at detecting drugs.

"The dog couldn’t find it and the officer also forgot which bag he put it in," a customs office spokeswoman said. "If by some chance passengers find it in their suitcase, we’re asking them to return it."

The spokeswoman quoted the unnamed 38-year-old officer as saying: "I knew that using passengers’ bags is prohibited, but I did it because I wanted to improve the sniffer dog’s ability."

He was reprimanded by the head of customs at Narita, Manpei Tanaka, who said: "This case was extremely regrettable. I would like to deeply apologize."

The marijuana, which has a street value of $9,680, was in a metal box wrapped with newspapers. Japan strictly prohibits both hard and soft drugs, with people imprisoned for possession of even small amounts of marijuana.