Infertility is defined by the International Committee for Monitoring Assisted Reproductive Technologies as the incapacity to conceive after at least one year of regular, unprotected, and well-timed intercourse, 1 and has become a global problem faced by one out of five couples. Although approximately 50% of these cases are associated with male infertility, 2 the etiology of male infertility is multifactorial and the average incidence of unexplained male infertility is approximately 15%. 2 Furthermore, approximately 40% of cases were unexplained in a recent survey on infertility among Japan males. 3 Many studies have reportedly revealed the pathophysiologies of unexplained male infertility, but the causes of most cases remain unknown. Many studies on the effect of seminal oxidative stress (OS) on male fertile capacity have been reported since Aitken et al first reported reactive oxygen species (ROS) in washed human semen in 1987 using a chemiluminescence assay. 4 OS results from a disturbance of homeostatic balance between ROS production and antioxidant capacity in seminal plasma in human semen 5 (Figure 1). It is well known that a small amount of ROS is vital for the steps involved in the essential physiological response of fertilization-sperm maturation, hyperactivation, capacitation, acrosome reaction of sperm, and