Broadcaster Fuji Television said Ishiba was cancelling the trip because a planned meeting between NATO and a group of four Indo-Pacific nations (IP-4) would likely not take place, and because a meeting with US President Donald Trump was also unlikely, Reuters reported.
South Korea and Australia, which along with the US and Japan make up the IP-4, have also said their leaders would not attend the NATO summit meeting.
Foreign Minister Takeshi Iwaya will represent Japan, the source said, declining to be identified because the plan is not public.