To pursue the maximum or minimum value of an output signal with a simple constitution by holding both control and output signals to calculate the change value of them, multiplying these change value by each other and increasing or decreasing the number of output signals according to the positive and negative result of multiplication of both change value.

A 1st sample-and-hold circuit 2 fetches the control signals Vc at a proper sampling interval and holds these signal value. A 1st compactor 3 compares the output of the circuit 2 with the present signal Vc and calculates the change value ΔVc. A 2nd sample-and-hold circuit 4 holds the output signals Vout at a proper interval. A 2nd comparator 5 compares the output of the circuit 4 with the present signal Vout and calculates the change value ΔVout. A multiplier 6 multiplies the value ΔVc by the value ΔVout, and a control circuit 7 increases or decreases the number of signals Vc according to the positive or negative result Vx of the multiplication.