The F-box and leucine-rich repeat protein 10 (Fbxl10) gene encodes a protein that catalyzes demethylation of H3K4 and H3K36. In this study, we show the important roles of FBXL10 as a histone demethylase in sustainable sperm production using mice in which the JmjC domain of Fbxl10 was deleted (Fbxl10^{DeltaJ/DeltaJ}). In histological analysis, testis sections from 10-wk-old Fbxl10^{DeltaJ/DeltaJ} mice appeared normal. On the other hand, testes from 7-mo-old Fbxl10^{DeltaJ/DeltaJ} mice contained a greater ratio of seminiferous tubules exhibiting degeneration of spermatogenesis. Further analysis using an in vitro spermatogonia culture system, that is, germline stem cells (GSCs), revealed that Fbxl10^{DeltaJ/DeltaJ} GSCs expressed a significantly higher level of P21 and P19 mRNA, cyclin-dependent kinase inhibitors and also known as cellular senescence markers, than wild-type (WT) GSCs. Furthermore, the ratio of Fbxl10^{DeltaJ/DeltaJ} GSCs in G0/G1 phase was higher and the ratios in S and G2/M phases were lower than the corresponding ratios of WT GSCs, and the doubling speed of Fbxl10^{DeltaJ/DeltaJ} GSCs was significantly slower than that of WT GSCs. In addition to these in vitro results, an in vivo study indicated that recovery of spermatogenesis after a transient reduction in the number of testicular germ cells by busulfan treatment was significantly slower in Fbxl10^{DeltaJ/DeltaJ} mice than in WT mice. These data suggest that Fbxl10 plays important roles in long-term sustainable spermatogenesis via regulating cell cycle.