An ultrasonic probe is constituted by ultrasonic transducer elements arranged in a direction. A beam forming unit forms acoustic field sensitivity modulated in a direction substantially orthogonal to scan lines in a subject by focusing an ultrasonic wave on a sequence of sampling points existing along the scan lines, and applying delay and apodization on a transmission/reception signal of each of the ultrasonic transducer elements. A detecting unit detects displacements at each sampling point in a direction tangential to the scan lines and in a direction tangential to curves substantially orthogonal to the scan lines by arithmetic between reception signals for the sampling point at different time points. Each curve of the curves substantially orthogonal to the scan lines is 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 is substantially the same.