PURPOSE: To decrease the frequency of operation of a valve while making up-down variation in liquid level as small as possible and holding the liquid level within a specific range, and to prolong the life of the valve by performing specific control arithmetic and outputting its result as an operation signal.

CONSTITUTION: A control arithmetic means 3 detects the time width Δt of Δh variation in deviation on the basis of the signal from a liquid level deviation judging means 2, solves an equation MVn=MVn-1+(V/Hi)*(±Δh/Δt)*(1±α)*(100/FOmax), and updates an operation output quantity according to the result. In the equation, MVn is the current operation output value, Mn-1 the last operation output value, V the capacity of a tank, and Hi the height of the tank. Here, Δh is a constant value obtained by optionally and equally dividing a control zone, and plus when a measured value is in an upper control zone or minus when the measured value is in a lower control zone. Then FOmax shows the maximum amount of discharge from the tank, and α is represented as 0≤α≤1 and minus when the level measured value of the tank varies toward a dead zone or plus when the measured value varies away from the dead zone.