In FIG. 2 (A), a reception beam (102) is formed using a weighting function (112). A position of a peak of the weighting function (112) is set at the position of the reception beam (102). A reception beam (104) is formed using a weighting function (114), and a position of a peak of the weighting function (114) is set at the position of the reception beam (104). A reception beam (106) is formed using a weighting function (116) and a position of a peak of the weighting function (116) is set at a position of the reception beam (106). In this manner, the positions of the peaks of the weighting functions (112, 114, 116) are shifted in the receive aperture, to follow movements of the reception beams (102, 104, 106) caused by electrical scanning.