The reproductive ecology of the Japanese grunt, Hapalogenys Mucronatus, setodai in Japanese, inhabiting Hiuchinada Sea, the west part of the Central Seto Inland Sea, was investigated from March 2000 to November 2001. The changes of GSI (gonadsomatic index), HSI (hepatosomatic index) and the histological observation in the gonad in the Japanese grunt were examined. GSI for both sexes changed low from October to the next April when it was estimated as the resting season. From the histological observation of the ovaries in May, they had immature eggs, so this period was estimated as the pre-spawning season. In this season HSI values were high. In June GSI began to increase, and then GSI attained to the maximum value, 3.3 for male in June and 17.3 for female in July. GSI of both sexes fluctuated at high level from June to September. This period was estimated as the spawning season. Especially, most ovaries were occupied by matured egg tissue with empty follicles after ovulating during from July to August when it was estimated as the full spawning season. HSI of both sexes changed low. It was inferred that energy resource was saved in liver for reproduction. In September, empty follicles were observed in some ovaries. Then, egg degeneration occurred in most ovaries, so it was estimated as the post-spawning season. The examination of ovarian oocytes distribution presented that most matured oocytes appeared at about 0.7 mm in diameter, and then many 0.4-0.5 mm oocytes in diameter were observed in ovaries which had empty follicles. Accordingly, it was concluded that the Japanese grunt has the poly-modal distribution in ovarian oocytes and spawns them continuously immediately after maturing immature oocytes.