Long-term field studies have provided evidence of worldwide declines in vertebrate populations, including amphibians and reptiles. However, studies investigating the decline in snake populations are scarce. As obligate predators, snakes play important roles in local ecosystems and are in turn affected by their food resources. In the present study we investigated the seasonal abundance of snakes and their food habits in a rice paddy field and compared the results of the present study with those of a previous study that was conducted at the same site approximately 30 years ago. Our results showed that Elaphe quadrivirgata, a dietary generalist snake that had been one of the dominant species at the site, has drastically declined and that Rhabdophis tigrinus and Gloydius blomhoffii have remained as dominant species. Furthermore, in the present study, most of the stomach contents of R. tigrinus consisted of the Japanese tree frog, Dryophytes japonicus, whereas three frog species, D. japonicus, Rana japonica, and Pelophylax porosus porosus, had been the main prey of R. tigrinus 30 years ago. In addition, fewer individuals of R. tigrinus and G. blomhoffii contained food in their stomachs than 30 years ago. Our findings imply that a change in prey availability may have affected the guild structure of snakes at this study site, although further investigation is necessary to verify the reasons for the decline of the dietary generalist, E. quadrivirgata.