1. A method for calculating a distance between consecutive two image frames obtained by moving a probe over a target to be examined, wherein the probe transmits and receives ultrasound signals, comprising the steps of: a) producing a first main frame, a second main frame parallel to the first main frame, and a supplementary frame inclined at an angle with respect to the first main frame; b) creating a virtual frame parallel to the first main frame by using the first main frame and the supplementary frame, wherein the virtual frame intersects the supplementary frame; c) calculating a first correlation coefficient between the first main frame and the virtual frame; d) computing a second correlation coefficient between the first and second main frames; and e) calculating a first distance between the first main frame and the virtual frame to estimate a second distance between the first and second main frames by using the first distance and the first and second correlation coefficients.