1. An ultrasonic diagnostic apparatus comprising: an ultrasonic probe configured to transmit/receive ultrasonic waves/echo signals to/from tissue of an object to be examined; and an ultrasonic diagnostic system constructed at least in part of circuitry, memory and a display, to perform operations to: provide driving ultrasonic pulses to the ultrasonic probe for the transmission of the ultrasonic waves; amplify received echo signals from the ultrasonic probe; generate tomographic data based on the amplified received echo signals; convert the tomographic data to a displaying tomographic image based on a gray-scale; obtain an RF signal group corresponding to a scanning area of ultrasonic beams for a portion of a plurality of frames of the amplified received echo signals; perform correlation processes on a selected pair of frames of different compression quantity, and generate displacement frame data by measuring the displacement of respective measurement points in the scanned area; calculate strain/elasticity modulus using the displacement frame data; generate elastic image data based on the strain or elasticity modulus; convert the generated elastic image data to a displaying elastic image based on a color-scale; switch at least one of the displaying tomographic image or the displaying elastic image; display the at least one of the displaying tomographic image or the displaying elastic image from the switch; and control data transmission from a transmitting circuit to the display by an interface; select a reference region on the at least one of the displaying tomographic image or the displaying elastic image, and calculate an indexed elastic image frame data based on formed gradation of ratio between the strain of the respective measurement points and the strain of the reference region, so as to eliminate the influence of compression quantity of the indexed elastic image frame data, wherein the display displays an indexed elastic image based on the indexed elastic image frame data.