PURPOSE: To simplfy and stabilize an outside controlling circuit, by inserting a resistance in series into a reading-out/writing gate to a bootstrap loop.

CONSTITUTION: 9 of the diagram is a bootstrap loop section, 10 is a data loop section, 11 is a regulated power supply for controlling both loops in common, and 12 and 13 are constant-current sources. The constant-current source 12 is an auxiliary one to be used when a special level is converted. When the value of an additional resistance Ra inserted in series into the reading-out/writing gate to the bootstrap loop is set to R1+Ra=R2, I1=I2 is obtained by the power supply 11 and stabilized driving is performed. Therefore, it is not necessary to control individually by using variable resistances as in the case of conventional ones and the reading-out and writing can be controlled easily with a single constant- current source 13. Moreover, since the electric current I1' flowing through the bootstrap loop section at the time of high-speed switching is also R1+Ra=R2, the electric current cannot become larger than I2 and no large transient current flows as in the case of conventional ones. Therefore, the circuit operation is stable.