To provide a tank having such a separation mechanism that an objective high-quality uniform liquid can be obtained by efficiently separating the objective liquid from the liquid which is to be treated and contains an impurity component of high density in a main body of the tank.

Centrifugal force is imparted to the original heavy oil in a rotary vessel 15 by rotating the vessel 15 by a rotary shaft 16 of the separation mechanism 12 in the main body 11 of the tank 10 and the centrifugal force-imparted original heavy oil is spouted from a nozzle 15A so that the original heavy oil is separated by a density difference into the heavy gravity heavy oil of high density containing the majority of the impurity component and the light poor heavy oil of low density containing a small amount of the impurity component. The obtained light poor heavy oil is furthermore filtered with a filtration mechanism 18 to obtain the high-quality poor heavy oil in which almost all of the impurity component is removed. The obtained heavy gravity heavy oil is cooled by a cooling coil 20 so that and the heavy gravity heavy oil is separated surely from the light poor heavy oil by the density difference.