1. A method for displaying in real time a stream line of an inhomogeneous flowing medium by tracing motion of speckles appearing in tomograms taken sequentially and separated by a predetermined time interval, said method comprising the steps of: (a) scanning an object in which said inhomogeneous flowing medium is situated by sending out pulses of an ultrasonic wave beam to scan the object plural times within a predetermined time interval; (b) forming images of the speckles by combining a first echoe of one of said speckles with a second echo of said one of said speckles taken after said first echo but within said time interval; and (c) obtaining graphic images of a trajectory of said speckles by producing and displaying an image corresponding to a difference between the two echoes of the same speckle.