A delay circuit of ultrasonic diagnostic apparatus comprising a delay line consisting of a plurality of unit portions connected in cascade, in each of which an anode of a first variable capacitance diode and a cathode of a second variable capacitance diode are connected with a connecting point of two inductors connected in series, or a delay line obtained by replacing capacitances in a second order all pass type delay line by variable capacitance diodes, or a delay line, in which variable capacitance diodes and inductors form a balanced circuit; and control circuit for applying a reverse bias voltage to each of the variable capacitance diodes; and an amplifier or a variable resistance circuit using FETs serving as a matching circuit for the delay line, capable of varying continuously an input impedance by using a control voltage from the exterior.