PURPOSE: To facilitate the setting of a focal length at a plurality of positions by arranging a circuit to generate a lump wave synchronous with the transmitting point of time, a resistance network section to generate a reference level and a comparator section which compares the lump wave with the reference level to output m signals to realize a transmission electronic focus.

CONSTITUTION: A voltage VAB is supplied to a circular resistance network 120 with a node A selection circuit 121 and a node B selection circuit 122. An output SX is inputted into an arithmetic circuit 130 and compared with a voltage of VMD=VAB/2 to output an AX. The output is compared with a lump wave VLMP by means of a comparator 140 and inputted into a pulser 110 as trigger signal TX given a delay time corresponding to a voltage value aX of the AX. In the pulser 110, vibrators are selected in a plurality of continuous channels with a shift register to transmit an ultrasonic beam to a subject.