Union members comprised an estimated 17.9 percent of employed workers in Japan as of June 30, down 0.2 percentage point from a year earlier, and a record low, the labor ministry said, noting the government started collecting such data in 1947.

While overall unionized workers declined 68,000 to 9,892,000, part-time workers in unions increased 61,000 to 837,000, accounting for 6.3 percent of all such workers, the ministry said Tuesday. Both the sum and the percentage figure were record highs.

"A factor behind the drop in union members is a rise of nonregular workers and a decline in regular workers amid the deteriorating economy," said Shuichiro Sekine, an official with Haken Union, a group representing temp-staff workers.

Sekine called for research into unionization as "many part-timers are paying union fees without knowing they belong to one."

Among labor union umbrella organizations, the Japanese Trade Union Confederation (Rengo) had the most members, with 6,693,000, down 7,000.