Japan on Monday lodged a diplomatic protest with Russia over the detention of a Japanese citizen visiting an island disputed by the two countries.
Tokyo and Moscow are working to resolve decades of tensions over four islands occupied by the Soviet Union in the closing days of World War II and now controlled by Russia.
The man, who has not been identified, was visiting one of the islands as a member of a visa-free visit programme that started in 1992, and was detained following baggage checks conducted ahead of his scheduled departure, Chief Cabinet Secretary Yoshihide Suga told reporters.
Tokyo "strongly protested through diplomatic channels" and demanded he be allowed to return home, said Suga, the government's chief spokesman.
Ties have been strained for decades over the islands, known in Japan as the Northern Territories and in Russia as part of the Kuril Islands.
The dispute has prevented the two countries from signing a peace treaty to formally end wartime hostilities.
The Russian government had no immediate public reaction to Japan's protest but its customs service said Monday on its website that "money was found" during a customs check of a Japanese citizen's luggage.
It said the amount came to four million Japanese yen, which is about $40,000.
"The said amount of money had not been declared to the customs agency as required by law," it said, adding that customs officials are looking into the "illegal action".
Under Russian law, a total of $10,000 can be taken out of the country undeclared.
The detention comes as Japanese Prime Minister Shinzo Abe and Russian President Vladimir Putin agreed in May to rekindle talks aimed at resolving the territorial dispute.
On Friday in Moscow, Chikahito Harada, Japan's special envoy for Japan-Russia relations, and Russian Deputy Foreign Minister Igor Morgulov will hold their second round of talks on the issue since the May bilateral summit.
Japanese media have reported recently that Abe and Putin will hold talks early next month in the Russian city of Vladivostok, though Tokyo has made no formal announcement.