1. An ultrasonic diagnostic apparatus, comprising: an ultrasonic probe which transmits/receives ultrasonic waves to/from an object while being in contact with the object; a transmission/reception unit which, in a process in which pressing force applied to the object by the ultrasonic probe is changed and a cross-sectional position for transmitting/receiving ultrasonic waves to/from the object is moved in a short axis direction, periodically transmits/receives ultrasonic waves to/from the object, performs reception processing on a reflection echo signal from the object, and measures RF signal frame data at the cross-sectional position; a displacement measurement unit which obtains a displacement of a living tissue at the cross-sectional position and sequentially generates displacement frame data on the basis of two pieces of RF signal frame data within a set time range, selected from among a plurality of pieces of RF signal frame data sequentially measured by the transmission/reception unit; an elasticity information calculation unit which obtains elasticity information of the living tissue at the cross-sectional position and sequentially generates elasticity frame data on the basis of the displacement frame data sequentially generated by the displacement measurement unit; and an elasticity image construction unit which sequentially constructs elasticity images on the basis of the elasticity frame data sequentially generated by the elasticity information calculation unit.