1. An ultrasonograph provided with a means for sending an ultrasonic beam into body tissue, a means for receiving an ultrasonic signal that has been reflected off the body tissue and a blood flow, a means for constructing a cross-sectional image of the body tissue from the received ultrasonic signal, a means for performing phase detection on an ultrasonic Doppler blood flow signal that has been reflected off the body tissue by the blood flow, a means for calculating a frequency component of the Doppler blood flow signal on which phase detection has been performed, and a means for performing a sweeping display of the calculated frequency component as Doppler images in a time series, the ultrasonograph having a heart rate measuring means for measuring a heart rate of a test subject at a predetermining timing, and the ultrasonograph comprising: an optimum sweep rate calculating means for calculating an optimum sweep rate based on the heart rate obtained by the heart rate measuring means; and an optimum sweep rate setting means for setting a sweep rate of at least either of the Doppler images and M-mode images.