1. An ultrasonograph, comprising: 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 performing phase detection on the ultrasonic signal received by the means for receiving to acquire an ultrasonic Doppler signal; a means for eliminating a low frequency component generated by moving tissue from the ultrasonic Doppler signal to acquire an ultrasonic Doppler blood flow signal; a means for calculating a frequency component of the ultrasonic Doppler blood flow signal; a means for performing a sweeping display of the calculated frequency component as Doppler images in a time series; and a processor comprising: a Doppler tracing means for generating a Doppler trace waveform by tracing maximum flow velocity points in the Doppler images; a heart rate measuring means for measuring a heart rate of a test subject at a predetermined timing based on the Doppler trace waveform; a sweep rate calculating means for calculating a sweep rate based on the heart rate obtained by the heart rate measuring means; and a sweep rate setting means for setting the sweep rate of the Doppler images based on the sweep rate calculated by the sweep rate calculating means, wherein the heart rate measuring means calculates a time required for each heart beat from the Doppler trace waveform obtained by the Doppler tracing means, and then calculates a heart rate per unit time based on the time required for each heart beat.