The mating system of the treefrog, Hyla japonica, may be typified as a lek in which males are grouped together and produce advertisement calls to attract females. The hotspot hypothesis states that males form a lek at the hot spot where most receptive females are mostly likely to be encountered. We tested the hotspot hypothesis with H. japonica by monitoring the number of male and female frogs in rice paddies weekly. Males of H. japonica occur in a complex of rice paddies, which is an assemblage of contiguous rice paddies for ease of irrigation and management. We divided the rice-paddy complex into 14 sectors that were composed of adjacent rice paddies with a similar water level. Lek was defined as the sector with the highest density of males in each week. When all rice paddies were suitable for spawning, the location of a lek shifted each week. Furthermore, the highest density of females did not occur in the lek, rejecting the hotspot hypothesis. At the beginning and end of the breeding season when only some rice paddies were filled with water suitable for breeding, the lek was formed at sectors with high levels of water. At the beginning of the breeding season, the location of a lek was matched with the sector with a highest density of females but at the end, it was not. In summary, the hotspot hypothesis was rejected in H. japonica when all rice paddies are full of water but when only some of paddies are, it is supported by the result only at the beginning of the breeding season. This result shows that the agricultural methods in rice paddies can affect the mating system of H. japonica.