To divert a return signal out of a linear operation range of a variable resistance circuit when the level of the return signal is large in controlling the gain in a preamplifier of an ultrasonic diagnostic apparatus by transmitting the return signal in the variable resistance circuit and varying the input resistance of the preamplifier.

In a receiving circuit with the preamplifier for amplifying a receiving signal from an ultrasonic vibrator, a buffer 30 and the variable resistance circuit 22 are mounted on a return passage 102. The input resistance of the preamplifier is varied according to the variation of the resistance value of the variable resistance circuit 22, by which the gain is adjusted. A voltage dividing circuit (a return signal level adjusting circuit) 31 keeps the level of the return signal within the linear operation range of the variable resistance circuit 22, by which distortion of the return signal generated there can be prevented.