An image showing the spatial distribution of the hardness of a tissue of an object to be examined from which the influence of the pressure amount is eliminated is displayed.From ultrasonic tomography data measured by pressing the tissue, a physical quantity relating to the distortion of the tissue at measurement points of the tissue is determined.An elasticity image of the tissue is created according to the physical quantity.The physical quantities at the measurement points are indexed by using the physical quantity in a reference region determined in the elasticity image as a reference, and an indexed elasticity image showing the distribution of the index value is created.