An ultrasonic probe (11) is constituted by a group of ultrasonic transducer elements arranged in at least one direction. A beam forming unit (91) forms acoustic field sensitivity modulated in a direction substantially orthogonal to a group of scan lines in a subject by focusing an ultrasonic wave on a sequence of sampling points existing along the group of scan lines, and applying delay and apodization on a transmission/reception signal of each of the ultrasonic transducer elements. A detecting unit (90) detects displacements at each of the sampling points in a direction tangential to the group of scan lines and in a direction tangential to a group of curves substantially orthogonal to the scan lines by arithmetic between reception signals for the sampling point at different time points. The group of curves substantially orthogonal to the group of scan lines is a group of curves in the subject, each of the curves being constituted by a sequence of points for which a total of times taken for ultrasonic pulses to reach from two points fixed to the ultrasonic probe (11) is substantially the same.
