1. An ultrasonic diagnostic apparatus comprising: an ultrasonic probe configured to transmit/receive ultrasonic waves to/from an imaging target; an ultrasonic transmitter/receptor configured to transmit ultrasonic waves, via the ultrasonic probe, and perform signal processing on reception signals from the ultrasonic probe; an ultrasonic image constructor configured to chronologically construct ultrasonic images of a chronologically measured imaging target, based on the reception signals that are processed in the ultrasonic transmitter/receptor; a display configured to chronologically display the chronologically constructed ultrasonic images; a controller configured to control the ultrasonic transmitter/receptor, the ultrasonic image constructor and the display; an operator configured to specify a time division of a staining commencement time when a contrast agent enters, for each pixel of the chronologically constructed ultrasonic images; an image analyzer in the controller configured to: determine a color assignment according to the time division of the staining commencement time, and generate a contrast progress image by applying a color to the pixel of the ultrasonic images with the determined color assignment, wherein the image analyzer comprises: a luminance analyzing section configured to: specify a staining commencement time at which an inflow of the contrast agent reaches a predetermined luminance distribution by analyzing a luminance value of the chronologically constructed ultrasonic images, extract a maximum value l max of a luminance in each pixel in an analysis range, obtain a value l α by multiplying the l max by a predetermined set threshold α, set a threshold value luminance attainment time t α , wherein the threshold value luminance attainment time t α is a time that the luminance reaches the value l α , and record the threshold luminance attainment time t α obtained in each pixel as a luminance analysis result, and a staining section configured to apply a predetermined color to the ultrasonic images that vary with time which are obtained after the specified staining commencement time; and an analysis frame setter configured to set the analysis range in which the luminance analysis is performed by the luminance analyzing section.