The small-scale sillago Sillago parvisquamis (family Sillaginidae) has been evaluated as one of the endangered species in Japan. The only largest local population exists at Nakatsu tidal flat in the southern part of the Suo-nada Sea (Buzen-kai Sea), Seto Inland Sea. The population structure of the species on the tidal flat was investigated in 2016-2018 to determine recent population trends. As a result, no dominant year class was observed in the population. Catch per unit effort (CPUE) (indiv./3 hours/person; mean ± SD) by rod and line fishing during the spawning seasons in 2016, 2017 and 2018 showed low values, at 0.80 ± 1.3 (n = 13), 4.3 ± 2.9 (n = 18) and 0.33 ± 0.65 (n = 9), respectively. Histological examination of ovaries of the females captured in 2018 indicated that spawning had occurred as yet. These findings suggest that the small-scale sillago stock has decreased to a critical level in Japan.