Information about the density of fish eggs is important to understand the vertical distribution of eggs and survival in early stage, in particular change in egg density is one of major issue in fisheries. This paper describes a practical application of an experimental system for measurement of the fish egg density under laboratory conditions. The device can control the temperature range in each water column, and make different densities at each layer. The density of olive flounder (Paralichthys olivaceus) eggs have ranged from 1,018.49 to 1,020.93 kg/m3 and were aggregated around the density of 1,020 kg/m3. The results show that the device is applicable for measuring the fish egg density under laboratory conditions.