Behavior patterns associated with spermatophore transfer have been studied in several species of insects, but in stoneflies, even the existence of a spermatophore has been uncertain. This study aimed to confirm the existence of spermatophores in Stavsolus japonicus (Plecoptera: Perlodidae) and to determine the manner of spermatophore transfer. The male bent its body into an S-shape and inserted its abdominal caudal tip between the sternites and the subgenital plate of the female to open the female's subgenital plate. After approximately one hour, the male started to stroke the female's eighth and ninth genital segments with its epiproct. Next, the aedeagus was extruded between the male's eighth and ninth sternites and a spermatophore-like mass was extruded from the aedeagus. The mass contained sperm and was thus considered to be a spermatophore. The spermatophore was placed directly under the female's subgenital plate by the aedeagus and the male then patted the spermatophore with his cerci.