In many animals, males increase their reproductive success by fertilizing as many females as possible. However, the number of females a male can fertilize depends on the intensity of male competition and the amount of sperm the male has. Especially, recent studies have shown that sperm production is more costly than previously expected. Tetranychus urticae Koch is a spider mite infesting various crops, and famous as a cosmopolitan pest. In the mite, the number of females with which a male mates may be limited by the intense male competition. In the absence of rival males, males mate with many females and, surprisingly, continue to mate even after sperm depletion. Mating without sperm transfer is considered costly because it takes time and energy whereas it produces no offspring. In this study, we hypothesized that males continue to mate because sperm is replenished in a very short period, and investigated the time to sperm replenishment in the mite. Results showed that sperm was replenished within 3h, supporting the hypothesis. Besides, the difference in mating duration suggested that males are able to recognize the amount of sperm remaining.