PURPOSE: To prevent a change in a propagation delay time and to realize the convergence of beam reduced in a side lobe as desired, by connecting a constant voltage source to the base of a bipolar transistor, wherein the drain of MOSFET is connected to an emitter and drive voltage is supplied to a collector through a resistor, through a resistor.

CONSTITUTION: In order to make the voltage Vds between a drain and a source constant regardless of the magnitude of drive voltage Vdd, a bipolar transistor Q4 is connected between MOSFET Q3 and a resistor R2. The drain of MOSFET Q3 is connected to the emitter of the bipolar transistor Q4 and, when the collector Q4 and the base thereof are connected to an amplifying stage Vc through resistors R2, R3, emitter potential is determined by base potential and almost equal to Vc and the voltage between the drain and source of Q3 becomes constant. Even when the drive voltage given to each driver is made different corresponding to a weighting value when the weighting of the transmission intensity at every element is performed in order to reduce a side lobe, propagation delay becomes the same value in all of drivers and delay difference exerting effect on the convergence of beam and fan-shaped scanning is not generated.