Provided are a sound speed calculation system and a sound speed calculation method that can calculate a sound speed in a region of interest of a subject with high accuracy. A focus (F) of ultrasonic waves (w) is set in a portion that is farthest in the transmission direction of the ultrasonic waves (w) in a region of interest (35). Several second ultrasound transducers (51) to (69) receive ultrasound echoes from the focus (F). A sound speed in the region of interest (35) is calculated using ultrasound echo signals output from a first ultrasound transducer (60) that is closest to the focus (F), two second ultrasound transducers (51) and (69) that are other than the first ultrasound transducer, are located on straight lines (L2) passing through the focus (F) and the region of interest (35) of the subject, and have the first ultrasound transducer (60) interposed therebetween, and other ultrasound transducers (52) to (59) and (61) to (68) between the first ultrasound transducer (60) and the second ultrasound transducers (51) and (69).
