TOKYO, JAPAN: An unsuspecting passenger who flew to Tokyo is carrying one million yen's worth of cannabis compliments of customs authorities after a sniffer dog failed a test, officials said on Monday (May 26).

An officer at Narita International Airport on Sunday stuffed 142 grams of the drug into the side pocket of a randomly selected black suitcase coming off an overseas flight so that the animal could detect it.

"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," she said.

The 38-year-old officer was quoted by the spokeswoman 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.

"This case was extremely regrettable. I would like to deeply apologise," said the airport's customs chief Manpei Tanaka.

The cannabis, which has a street value of one million yen, 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 cannabis.