In quantitative analysis using blood flow velocity measurement and the like, whether to select the forward direction or reverse direction is determined in accordance with the positional relationship between a baseline for controlling the display scale of a Doppler waveform and a reference line which can be arbitrarily set by an operator. The determination is executed at one of the stages of selecting a tracing direction by an automatic tracing process, selecting an approximate Doppler waveform as an analysis target, and selecting an analysis result as a display target.