1. An ultrasound diagnostic apparatus comprising: a transducer array; a transmitter which supplies an actuation signal to the transducer array to transmit an ultrasonic beam from the transducer array toward a subject; a receiver which processes a reception signal output from the transducer array having received an ultrasonic echo by the subject to acquire reception data; a transmission/reception controller which controls the transmitter and the receiver such that, when n is an integer equal to or greater than 3, reception data of n frame images is sequentially repeatedly acquired in order to synthesize n frame images having different steering angles of the ultrasonic beam to produce an update image, and the angle difference in steering angle between two frame images, for which the acquisition of reception data is most temporally separated, among the n frame images for use in producing the update image is smaller than a maximum angle difference among the angle differences in steering angle between two frame images among the n frame images; and an update image producer which produces each update image based on reception data acquired by the receiver, wherein, when m is an integer equal to or greater than 1 and less than n, each time reception data of m frame images is acquired by the receiver, the update image producer synthesizes the previous n frame images, for which reception data is sequentially acquired hitherto, to produce the update image.