Inflation-adjusted real wages, a barometer of consumer purchasing power, fell 1.4 percent from the same month last year, according to the Ministry of Health, Labor and Welfare.
Widening from a revised 1.2 percent fall in April, the decrease came amid higher materials costs and a weaker yen which pushed up import costs, the data showed.
Nominal monthly earnings, including base pay and overtime, rose 1.9 percent from a year ago to reach 297,151 yen (about 1,850 U.S. dollars).
Regular pay including basic salary in May went up 2.5 percent year on year, while overtime pay, a gauge of business activity, went up 2.3 percent.
Meanwhile, the consumer price index excluding imputed rent, used to calculate the real wage index, grew 3.3 percent year on year, the preliminary data showed.
Separate data released by the Ministry of Internal Affairs and Communications on Friday showed Japan's household spending in May saw the first decline in two months by shrinking 1.8 percent from a year earlier.
Analysts here noted that with prices at historically high levels and real wages falling for the past two years, people's lives are becoming increasingly tight and the recovery of personal consumption spending may take longer than expected.
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