PURPOSE: To relieve a trouble of selecting a time constant depending on the load state and to attain high circuit integration by controlling automatically the operation of an emitter follower or a source follower depending on the load charge/discharge.

CONSTITUTION: A resistor 1 is added between a 1st power supply Vcc and one terminal of a switching section connecting to resistors 2, 3 in common, and resistors 7, 12, 13 and a diode 11 are provided in a logic output section. An output logic signal and a signal with an inverted phase are detected by a resistor 7 connecting between the power supply Vcc and the collector of the emitter follower TR 8 and the current flowing to the constant current source circuit of the emitter follower TR 8 is controlled to increase the charge/ discharge capability of the emitter follower. While the output logic signal keeps changing, a signal controlling the constant current source circuit is being generated. Thus, it is not required to make processing such as arrangement of time constants of many kinds or automatic switching with a CAD or the like and the deterioration in the circuit integration degree is prevented.