An imaging part scans a predetermined site of a subject to which a contrast agent has been administered, with ultrasound waves, and thereby acquires plural frames of ultrasound image data. A contrast agent inflow detector detects the inflow of the contrast agent into each region based on the signal intensity in each region represented in the plural frames of ultrasound image data. A reference time determining part determines, as a reference time, a time point when the contrast agent inflow detector detects the inflow of the contrast agent into a first region of interest. A color coding part obtains a relative time of the inflow of the contrast agent into each region with reference to the reference time, and generates image data representing the color of each region with a hue corresponding to the relative time. An image based on the image data representing the hue is displayed.