PURPOSE: To realize the medical ultrasonic diagnostic device for measuring exactly an ultrasonic sound speed of the human body.

CONSTITUTION: The ultrasonic diagnostic device 2 is constituted so that its set speed can be varied stepwise by using a duplex probe 1 consisting of a single element and a linear array, and the ultrasonic diagnostic device 2 which can drive these probes, and by deriving such a set sound speed as an echo signal is obtained when a gate of a cursor 6 is aligned with the surface of an echo image 5 on an image display part 3 of the ultrasonic diagnostic device 2, this set sound speed is set as a sound speed of a body to be examined, which is being diagnosed.