1. A method for estimating acoustic velocity of an ultrasonic image, comprising: (A) dividing each of the ultrasonic images of a plurality of input frames into a plurality of blocks; (B) extracting contours of the ultrasonic images corresponding to each of the plurality of blocks for each of the frames; (C) calculating and analyzing average luminance values of each of the blocks for each of the frames in order to select optimal blocks in a sequence of blocks having a maximum luminance value among average luminance values of each of the blocks when the contour extraction for each of the blocks is completed; (D) determining the number of optimal blocks according to the average luminance of each of the frames by calculating the average luminance values of each of the frames using the average luminance values of each analyzed block and selecting the optimal blocks as many as the number of optimal blocks; and (E) selecting, as an optimal frame, a frame corresponding to the optimal block having a maximum luminance value among the optimal blocks of each frame after comparing the selected optimal blocks with the optimal blocks of the remaining frames and estimating and applying acoustic velocity of the optimal frame as the real acoustic velocity, wherein step (D) includes: (D-1) calculating the average luminance values of each of the frames by using the average luminance values of each analyzed block, (D-2) determining the number of optimal blocks according to the average luminance values of the frames, and (D-3) selecting the optimal blocks as many as the number of optimal blocks in an order of the maximum luminance value among the average luminance values of each block.