The disclosed invention may accurately obtain an ambient sound velocity without causing degradation in the processing or diagnostic efficiency. Transmitting an ultrasonic wave focused on a predetermined transmit focus position by driving each of the elements of the ultrasonic probe based on a predetermined transmit delay time, and determining a true focal position of the ultrasonic wave transmitted to the transmit focus position or a focal point valid region that includes the true focal position based on a receive signal received by each element according to a reflection wave reflected by the transmission of the ultrasonic wave to the transmit focus position.
