PURPOSE: To mix liquid by stable temperature gradient in such a way that pressure shocks which are produced due to opening and closing of electromagnetic valves by installing a gas buffer on the downstream path of the electromagnetic valves to open and close a liquid reservoir in a liquid feeder in the gradient method.

CONSTITUTION: Respective liquid reservoirs containing different liquids are conducted through electromagnetic valves 1, 1' via an upper spacer 2 to a mixer 5. The mixer 5 is composed of outer pipings 3, 3', the upper spacer 2 and a lower spacer 2' which slide therein. A gas buffer has a space 7 which is formed by an outer wall 8 and a spacer 9, and the space 7 is connected through a connecting pipe equipped with a three-way cock 6 to the mixer 5. A plunger pump is connected to an outlet pipe 10. Mixed liquid which is continuously varied by the mixer is formulated by gradually and continuously varying the opening and closing time of the electromagnetic valves. In this case, the pressure shocks are buffered by the gas buffer.