Households of two or more people spent an average of 290,328 yen (about 1,800 U.S. dollars), reversing the 0.5-percent rise in April, according to the Ministry of Internal Affairs and Communications.
On a seasonally adjusted basis, spending fell 0.3 percent from the previous month.
Per workers' household in May, the average monthly income was up 3 percent in real terms from the previous year, with a 1.1-percent drop in spending, the ministry report showed.
"It is still uncertain whether wages have kept pace with rising prices," a ministry official was quoted as saying by national news agency Kyodo on Friday.
By category, food expenditure, accounting for around one-third of household spending, shrank by 3.1 percent year-on-year in real terms, as people were spending less on vegetables and cooked food during the reported period.
Outlays for culture and recreation fell 8.4 percent in May due to lower spending on overseas tour packages affected by the yen's persistent depreciation against other major currencies.
Accounting for more than half of Japan's gross domestic product, household spending is a key gauge of private consumption in the country.
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