PURPOSE: To provide a constant flow liquid level controller which lightens or excludes trial and error operation at the time of determining the PI control parameter of a control calculating means, and in which the control parameter to realize the optimum trade-off between requests opposed to each other, i.e., constant flow and liquid level control is set.

CONSTITUTION: In a constant flow liquid level controller provided with the control calculating means 15 to determine a flow rate adjust signal by PI control calculation from a fluid level detection signal and a liquid level set value signal in a fluid storage tank 11, when the sectional area of the fluid storage tank 11 is made A, fluid level meter gain is made Km, and flow rate controller gain is made Kv, proportional gain K and reset time T in the control calculating means 15 are set so as to satisfy the relation of KT=2Tp provided that Tp=A /(Km.Kv).