To clarify the spawning area of robust tonguefish Cynoglossus robustus in Osaka Bay, we recorded the location of the fishing grounds via board gill net fishing boats operating during the spawning season, and identified the high-catch sea area during the spawning season using the operation logbooks of small dredge net fishing boats. Our study revealed that the spawning area of this species is located along the contour line at a depth of 30 m, extending in the north–south direction in the central part of Osaka Bay. Additionally, data suggests that spawning occurs from around sunset to nighttime, because catching of females that discharge absorbent eggs in the gill net operation started about 2 h before sunset, and about half of the mature females caught after dusk discharged absorbent eggs.