Ultrasonic transmission and reception for sound speed setting is performed in response to an instruction to freeze, an instruction on an observation target range, an instruction to change image quality and an instruction to change an image mode, a reception signal obtained by the ultrasonic transmission and reception for sound speed setting is used to set the sound speed in a subject, and a reception signal obtained during ultrasonic transmission and reception is processed based on the set sound speed to produce an ultrasound image. Such method makes it possible to produce a high-quality ultrasound image subjected to delay correction appropriate to various operations on an ultrasound diagnostic apparatus, such as freezing and change in image quality.