To provide an automatic liquid quantitative mixing device used in a method of automatically and quantitatively mixing liquid by which a liquid mixture of prescribed concentration is accurately adjusted irrespectively of physical properties such as viscosity of the liquid mixture.

Liquid A is introduced into an inner vessel 3 and the fixed quantity thereof preset in advance is detected by a measuring sensor 4 to stop the introduction of the liquid A. Liquid B is introduced into an outer vessel 1 incorporating the inner vessel 3 and also is caused to flow into the inner vessel 3 to mix it with the liquid A. The liquid mixture of the liquid A and the liquid B having reached a fixed quantity in the outer vessel 1 is detected by a quantitative sensor 2 to stop the introduction of the liquid B, thus allowing automatic liquid quantitative mixing to be performed.