The pace of fall in the core consumer price index, which includes oil products but excludes volatile fresh fruit, vegetable and seafood prices, was faster than a median market forecast for a 1.4 percent decline.
The fall widened as the government has abolished public high school tuition fees from April to support householders, which pushed down annualised core CPI by 0.54 percentage point, the Ministry of Internal Affairs and Communications said.
Discounting such technical factors, analysts say the overall price trends are unchanged, with deflation gradually easing as the economy has been recovering since April-June last year after suffering its worst slump in many decades.
The so-called core-core inflation index, which excludes food and energy prices and is similar to the core index used in the United States, fell a record 1.6 percent from a year earlier, due in part to the scrapping of high school fees. It had tumbled a record 1.2 percent in January and December.
Core consumer prices in Tokyo, available a month before the nationwide data, slid 1.6 percent in May from a year earlier, after a 1.9 percent drop in April, and compared with a market forecast for a 1.5 percent decline.