In many sexually reproducing organisms, males can produce more offspring by mating more often, but for females, increased mating frequency does not necessarily lead to more offspring. This mismatch of interests between the sexes can lead to sexual conflict, which can develop into an evolutionary arms race. Diving beetles are a particularly interesting example. Many diving beetles mate underwater but have to surface every few minutes to breathe. During mating, the male attaches a sucker to the female's back to prevent her from escaping. At this time, the female is positioned below the male, making it difficult for her to surface. As a result, prolonged mating by males with developed suckers leads to an increased risk of suffocation and predation. Previous studies have suggested that females have evolved a rough surface on their dorsal surface to reduce the stickiness of male suckers. However, some species with male suckers show no sexual dimorphism on their dorsal surfaces, and the nature of sexual conflict in the diving beetle remains unclear. In the diving beetle (Eretes griseus), males have well-developed suckers (two large suckers and many smaller suckers on each tarsus of the foreleg), whereas females show no dorsal sexual dimorphism. In this study, the mating behavior of E. griseus collected in Kagawa Prefecture was observed, and the relationships between male suckers and mating traits, as well as female counteradaptation, were analyzed. The results revealed that the number of small suckers was positively correlated with the number and duration of mating but not with the size of large suckers. This means that in E. griseus, small suckers may be more important for mating success than large suckers are. Females also had significantly longer hind legs and were more likely to kick males during mating. Furthermore, the greater the number of small suckers and the shorter the hind legs of the female are, the more successful the mating and the longer the mating period. This finding suggests that in E. griseus, there is an arms race between the number of male suckers and the length of female hind legs due to sexual conflict.