Methane hydrate has been pald much attention as a new natural resources and an alternative energy for crude oil or liquefied natural gas, and huge amount of methane hydrate was distributed under the deep seabed in the offshore of Japan. The knowledge of density of clathrate hydrate is essential for recovering natural hydrate resources. Then, a new flow type apparatus for the density measurement of hydrate slurry was developed. An oscillation U-tube densimeter was equipped with the apparatus. In the measurements, water and liquefied guest molecule were continuously supplied by two pumps to mix up liquid-liquid equilibrium like emulsion. In pre-cooling line, the phase transition was occurred, and the slurry, aqueous solution + hydrate, was prepared. The slurry was successively sent into the oscillation U-tube densimeter. Then, the apparent density of the slurry can be measured. Prior to the measurements, plugging up concentration was estimated by use of phase diagram and percolation theory. Because of the formation of continuous solid phase at the percolation threshold, it causes the lost of fluidity and plugging up in the flow line. In our previous study, the apparent densities were successfully obtained for tetrahydrofuran (THF), propane and carbon dioxide hydrate slurry. In this paper, experimental procedure and model calculation were described for carbon dioxide slurry. The technique can be applied not only for the determination of the lattice parameter in clathrate hydrate, but for a sensor to avoid plugging up in boring tube or pipeline.