1. An ultrasound diagnostic apparatus comprising: a piezoelectric element array which has piezoelectric elements arranged therein, each piezoelectric element configured to transmit an ultrasonic wave, to receive an ultrasonic echo reflected by a subject, and to output a reception signal according to a received ultrasonic wave; a controller which controls ultrasonic transmission and reception by the piezoelectric element array; a storage which stores the reception signal output from the piezoelectric element array; a sound speed setter which sets a sound speed in the subject using the reception signal stored in the storage; an image producer which processes the reception signal output from the piezoelectric element array or the reception signal read from the storage, based on the sound speed set by the sound speed setter to produce an ultrasound image; a display; and an operating device which has a freeze instruction inputter configured to instruct to display a still image on the display, an observation target range instruction inputter configured to specify an observation target range of an ultrasound image displayed on the display, an image quality change instruction inputter configured to instruct to change image quality of the ultrasound image, and a mode change instruction inputter configured to instruct to change an image mode of the ultrasound image, wherein the controller causes the piezoelectric element array to perform ultrasonic transmission and reception for sound speed setting so as to allow the sound speed setter to set the sound speed in response to at least one instruction out of an instruction to display a still image from the freeze instruction inputter, an instruction on the observation target range from the observation target range instruction inputter, an instruction to change image quality from the image quality change instruction inputter, and an instruction to change an image mode from the mode change instruction inputter, the sound speed setter sets the sound speed in the subject using a reception signal obtained by the ultrasonic transmission and reception for sound speed setting to update the sound speed, the image producer processes the reception signal output from the piezoelectric element array based on an updated sound speed to produce an ultrasound image, and the display displays the ultrasound image produced by processing based on the updated sound speed.