To provide a method for pumping out liquid reserved in a container which does not cause the liquid from leaking from a gap between a seal member and an inner peripheral wall of the container even if the sealing force of the seal member around a follow plate is weakened, and which facilitates pumping-out operation so as not to require skill.

According to the method for pumping out liquid reserved in a container, the follow plate 10 is put into the container P of a pail can in which the low-viscosity liquid T is reserved, so as to be lowered by a lifting device in such a state that the seal member around the follow plate 10 is in contact with the inner peripheral wall of the container, and the reserved liquid is pumped out by driving a pump unit 2 while keeping the lowering of the follow plate 10 under such a condition that the follow plate 10 is seated on a surface of the reserved liquid. By substantially equally balancing downward load by the weight of the follow plate 10 per se and the load of the pump unit 2 with upward load by a spring balancer 38, the sealing force of the seal member 15 around the follow plate 10 with respect to the inner peripheral wall of the container P is weakened and the follow plate 10 is lowered according to an amount of the liquid pumped out by the self-priming force of the pump unit 2.