An ultrasonic signal processing method includes: measuring a sound velocity in a subject according to transmission/reception data acquired in ultrasonic transmission of M times (M is an integer equal to or greater than 1 and less than N) along different transmission focus lines among ultrasonic transmission of N times (N is an integer equal to or greater than 2) along multiple transmission focus lines in a case where ultrasonic transmission is sequentially performed at least one time on each of multiple transmission focus lines to create an ultrasonic image for one frame.
