A separation method, which is a method for continuously separating a light liquid and a heavy liquid having a specific gravity larger than that of the light liquid from a mixed liquid containing the light liquid, the heavy liquid, and an emulsion liquid of the light liquid and the heavy liquid by continuously introducing the mixed liquid into a separation tank, wherein the separation tank comprises: a tank body; a chamber A into which the mixed liquid is introduced and in which an interface between the light liquid and the heavy liquid is formed, a chamber B into which the light liquid flows from the chamber A by overflowing an overflow part (Wa), and a chamber C into which the heavy liquid flows from a bottom part of the chamber A via a liquid transfer pipe, which are chambers respectively formed by partitioning an inside of the tank body by an erected wall (W1, W2) erected from a bottom surface of the tank body; and an extraction part for an emulsion liquid configured to extract the emulsion liquid from the middle of the chamber A in a vertical direction, in which one end of the liquid transfer pipe is opened in the chamber A, and the other end thereof is opened in the chamber C, an upper end of an erected wall partitioning the chamber A and the chamber C is configured to be positioned higher than the overflow part (Wa), and wherein when a height from the bottom part of the tank body to the overflow part (Wa) is defined as h 1, a height from the bottom part of the tank body to an opening (Na) of the liquid transfer pipe in the chamber C is defined as h 2, a height from the bottom part of the tank body to the interface in the chamber A is defined as h 1 ·α, and a ratio P H /P L of specific gravity P H of the heavy liquid to specific gravity P L of the light liquid is defined as X, a relationship of α = (h 2 × X ― h 1)/h 1 (X ― 1) is established, α ≥ 0.2 when X is the minimum value X min, and α ≤ 0.7 when X is the maximum value X max, wherein the separation tank includes a detection part configured to confirm presence or absence of the emulsion liquid by detecting the height from the bottom part of the tank body to the interface in the chamber A, and the separation tank includes outflow parts (N2, P2, V2, L2; N3; N3, P3, L3) configured to allow liquids to flow out from the chamber B and the chamber C respectively so as to keep liquid levels in the chambers within a predetermined range. a tank body; a chamber A into which the mixed liquid is introduced and in which an interface between the light liquid and the heavy liquid is formed, a chamber B into which the light liquid flows from the chamber A by overflowing an overflow part (Wa), and a chamber C into which the heavy liquid flows from a bottom part of the chamber A via a liquid transfer pipe, which are chambers respectively formed by partitioning an inside of the tank body by an erected wall (W1, W2) erected from a bottom surface of the tank body; and an extraction part for an emulsion liquid configured to extract the emulsion liquid from the middle of the chamber A in a vertical direction, in which one end of the liquid transfer pipe is opened in the chamber A, and the other end thereof is opened in the chamber C, an upper end of an erected wall partitioning the chamber A and the chamber C is configured to be positioned higher than the overflow part (Wa), and wherein when a height from the bottom part of the tank body to the overflow part (Wa) is defined as h 1, a height from the bottom part of the tank body to an opening (Na) of the liquid transfer pipe in the chamber C is defined as h 2, a height from the bottom part of the tank body to the interface in the chamber A is defined as h 1 ·α, and a ratio P H /P L of specific gravity P H of the heavy liquid to specific gravity P L of the light liquid is defined as X, a relationship of α = (h 2 × X ― h 1)/h 1 (X ― 1) is established, α ≥ 0.2 when X is the minimum value X min, and α ≤ 0.7 when X is the maximum value X max, wherein the separation tank includes a detection part configured to confirm presence or absence of the emulsion liquid by detecting the height from the bottom part of the tank body to the interface in the chamber A, and the separation tank includes outflow parts (N2, P2, V2, L2; N3; N3, P3, L3) configured to allow liquids to flow out from the chamber B and the chamber C respectively so as to keep liquid levels in the chambers within a predetermined range.