The cause of differences in individual growth rates (uneven growth) of Japanese sea cucumber Apostichopus japonicus seedlings was investigated by changing the conditions of rearing density and food amount in a nursery culture experiment. As a result, when the animals were fed. some of the animals grew rapidly so uneven growth occurred regardless of the density. In other animals, the growth rate was closer to that of the non feeding condition, and it was suggested that the uneven growth might occur under the condition of the occurrence of some animals being satiated and others starving. Further, assuming such an unevenness of food amount was derived from the unevenness of opportunity to eat, the feeding method was improved, and the uneven growth was kept smaller than before (usual feeding method). Therefore, unevenness of the available food amount was considered to be the cause of the uneven growth of the sea cucumber seedlings.