Serious problems have developed in Japanese Black cattle (i.e., Tajima beef cattle): (i) inbreeding coefficients (ICs) are continuously increasing, and (ii) some sire candidates fail in the first reproductive examination for sire selection, due to poor sperm quality. To investigate possible causal factors for this poor sperm quality, we accumulated data concerning the ICs (0.14-0.35) and sperm characteristics of 168 sire candidates, and we then analyzed the relationship between the ICs and the sperm quality. We set the standards for good sperm quality as ≥ 80% of progressively motile spermatozoa (PMS) and ≤ 15% of morphologically abnormal spermatozoa (MAS) in fresh ejaculates and ≥ 40% of PMS in cryopreserved semen. The proportions of sire candidates meeting these standards were significantly negatively correlated with the ICs (p < 0.01) in each sperm-quality criterion. Moreover, we found an inbreeding class threshold (0.21 for F-PMS and 0.24 for C-PMS) at which the proportion of sire candidates meeting semen quality criteria changes, especially in F-PMS and C-PMS (p < 0.01). These results suggest that the increased ICs have caused the decreases in male Tajima beef cattle's reproductive ability to produce spermatozoa with normal motility, morphology, and freezing tolerance.