The Japanese giant box jellyfish Morbakka virulenta (Kishinouye, 1910) occurred in early fall to early winter in the western and central parts of the Seto Inland Sea, western Japan. We obtained young medusae to fully mature individuals (8.5~22.2 cm in bell height) in the fields. A relationship between the bell height (x cm) and wet weight (y g) were indicated as the following regression: y = 0.166x2.9504. Sex ratio (female: male) of mature individuals examined (n= 57) was 1:1.5, and not statistically significant.