1. An ultrasound diagnosis apparatus, comprising: an ultrasound probe; and a processor having a program stored therein for causing the processor to perform executable units, the units including: a change amount calculating unit configured to calculate an amount of change of a pattern of each of a plurality of local regions among a plurality of ultrasound images generated along a time series based on a reflected wave of an ultrasound wave transmitted by the ultrasound probe, wherein the plurality of local regions are set in each of the plurality of ultrasound images and each of the local regions has a width and is set with a predetermined spacing in a lateral direction; an estimating unit configured to generate a position of a rotation axis of a scanning cross-section in each ultrasound image based on the amount of change of the pattern of each of the plurality of local regions calculated by the change amount calculating unit, wherein the estimating unit is configured to generate a graph in which the amount of change of the pattern of each of the plurality of local regions is plotted, generate, using curve-fitting, an approximate curve from the plotted points of the graph, and estimate the position of the rotation axis based on the generated approximate curve; and a display control unit configured to superimpose the rotation axis estimated by the estimating unit on each ultrasound image and display each ultrasound image on a predetermined display.