Japanese police have arrested a Singaporean man after finding about four kilograms of illegal drugs hidden inside his wheelchair as he arrived at a Tokyo airport, reports said.

The unidentified 47-year-old man flew into Narita International Airport near Tokyo on September 5 on a Cathay Pacific flight from Hong Kong via Taipei, the Kyodo and Jiji Press news agencies reported.

Four bags containing unidentified stimulants worth about $US4 million were found inside the seat cushion of his wheelchair, the reports said, adding that prosecutors had since indicted the man.

A Japanese customs source was quoted by Kyodo as saying the case was "quite unconscionable because the suspect probably tried to take advantage of inspectors' attitude toward the disabled".

It was not immediately clear if the man was genuinely disabled or had used the wheelchair as a ruse.

- AFP