One of the jumbo jets used by Japanese Prime Minister Shinzo Abe for his foreign trips lost a wing panel during a flight.
A laptop sized panel fell off the Boeing 747 during a flight from Tokyo to its base in Hokkaido. The Prime Minister was not aboard the aircraft at the time of the incident.
The lost part, which measures 38cm by 20cm (15in by 8in), is an access panel near the pylon, connecting one of the engines to the right wing of the plane.
It is the latest in a string of mishaps where US aircraft based in Japan have lost parts.
Last month, a helicopter window fell and landed in a school playground.
The incident sparked public opposition to US aircraft and prompted the Japanese defence minister, Itsunori Onodera, to ask his US counterpart, James Mattis, to implement measures that would prevent anything similar happening again.
The Air Self Defence Force said the part lost in the recent incident has not been found. An investigation into the cause of the accident is under way.
Two government jumbos left Tokyo on Friday carrying Mr Abe and other officials and business leaders on a six-day trip to Eastern Europe, including stops in Estonia, Lithuania, Serbia and Romania.
Mr Abe did not board the 747 that lost the panel.
Two Boeing planes are to replace the ageing jumbos in March 2019.