an ultrasonic diagnostic apparatus has an ultrasonic probe for transmitting and receiving an ultrasonic wave at a group of a plurality of ultrasonic vibrators arranged in an array form to and from a subject and incorporating a preamplifier groups for amplifying the resulting weak ultrasonic echo received after obtaining tissue information about the subject . the ultrasonic vibrator group has a first area to which a first amplifier group is to be connected and a second preamplifier group to which a second amplifier group is to be connected . in an operating mode to cease the first preamplifier group from operating , a control circuit switches to supply the bias current being supplied to the first preamplifier group over to the second preamplifier group .