A diplomat at the Japanese embassy in the Democratic Republic of the Congo has been arrested on suspicion of starting a fire at the mission.
Shinya Yamada, 30, was held in Tokyo over claims he embezzled funds and set the blaze in June to destroy evidence.
Some $260,000 (£160,000) had gone missing, Japan's foreign ministry said. No-one was hurt in the fire, which damaged the building's fourth floor
Mr Yamada has denied the allegations, according to Japanese media reports.
It is rare for Japanese investigators to probe a case that occurred at a diplomatic mission abroad.
"It is extremely regrettable an official of the foreign ministry was arrested and that serious damage, including the loss of public funds, was inflicted," AFP news agency quoted Foreign Minister Fumio Kishida as saying.
Mr Yamada, the third secretary at the Japanese embassy in Kinshasa, was reportedly in charge of accounting at the mission.
Police believe he embezzled embassy money to fuel his gambling habit before dumping petrol in his office and setting it ablaze, reports said.
Records of those entering and exiting the embassy building showed Mr Yamada was the last person to leave on the day of the fire.
Police also found a petrol tank at Mr Yamada's home in Kinshasa.
After the fire, the mission's 20 staff were transferred to temporary offices at the ambassador's official residence and a nearby apartment complex, the Japanese foreign ministry said.
Mr Kishida said his office was "closely cooperating with the police investigation", adding an apology "to the people of Japan".