An ultrasound diagnostic apparatus according to an embodiment includes an image acquirer, a volume information calculator, a wall motion information calculator, a time change rate calculator, an extremum detector, and an index calculator. The image acquirer acquires ultrasonic image data including data for a left ventricle. The volume information calculator calculates time-series data of volume information of the left ventricle. The wall motion information calculator calculates time-series data of wall motion information of the left ventricle. The time change rate calculator calculates time-series data of the time change rate of volume information (first time-series data) and time-series data of the time change rate of wall motion information (second time-series data). The extremum detector detects extremums in early diastole of the first time-series data and the second time-series data (first extremum and second extremum). The index calculator calculates an index using the first extremum and the second extremum.