Tokyo (CNN)Drinks after work probably seemed like a good idea -- but for one worker in Japan the hangover is likely to last for some time.
The unnamed worker lost a USB flash drive containing the personal details of every resident of the city of Amagasaki, northwest of Osaka, after going for drinks this week, according to a statement Thursday from the city's government.
Public broadcaster NHK reported that the worker, a man in his 40s, fell asleep on the street after drinking alcohol at a restaurant. When he woke up, his bag containing the flash drive was gone.
The man works for a company tasked with providing benefits to tax-exempt households, the city government's statement said.
On Tuesday, he went to the city administration's information center and transferred residents' data onto a flash drive. The data included the names, birth dates, and addresses of 465,177 people, the statement said -- the city's entire population.
The flash drive also contained sensitive information including tax details, bank account names and numbers, and information on households receiving public assistance such as childcare payments.
On Wednesday, the employee searched for the flash drive but couldn't find it, so he filed a lost property report with the police, according to the government statement. Later that afternoon, the company notified city authorities about the loss.
The flash drive is encrypted, and no data leak has been confirmed yet, authorities said in the statement.
It added that though the employee had been authorized to access the data, he had not been given permission to transfer it onto a separate electronic device. The statement also criticized the employee for failing to erase the data from the flash drive after completing his work at the city office, and for carrying it personally instead of using a more secure transportation method.
Authorities held a news conference on Thursday, with the city's mayor and other officials bowing in apology to residents.
The city will make payments to eligible households without delays, and will provide more information on the case as they investigate, said the official statement -- which ended with a pointed reminder for government employees to get permission before taking flash drives out of the city's offices.