Young stands of Cryptomeria japonica were thinned at various intensities in experimental plots in Ibaraki, Kyoto, and Kumamoto Prefectures. The amount of male strobili produced in the first summer after the thinning was measured in each plot by using litter traps. Few male strobili were formed in Kumamoto, but in Ibaraki and Kyoto the amount of male strobili per unit land area, the proportion of trees producing male strobili, and the mean amount of male strobili per such tree were greater in the heavily and extra-heavily thinned stands than in the unthinned and normally thinned stands. The production of male strobili per unit land area appears to reach a maximum when Ry, the yield index, which is an index of relative stand density, is a certain value (below 0.4 in Kyoto and below 0.6 in Ibaraki). We conclude that normal thinning (by which the Ry of a closed stand is decreased by less than 0.15) is not likely to reduce male strobili production in sugi plantations after the thinning.