The medical image diagnostic device of the present invention is a medical image diagnostic device provided with a tomography image construction unit for constructing a tomography image of a subject body, a measurement unit for measuring cardiac measurement values on the basis of the tomography image; and an image display unit for displaying the tomography image and the cardiac measurement values; wherein the medical image diagnostic device is provided with a characteristic waveform detection unit for detecting a characteristic waveform from the shape of the ECG waveform of the subject body; and a cardiac period setting unit which treats as a first characteristic waveform a characteristic waveform which, among characteristic waveforms detected at the characteristic waveform detection unit, has the smallest difference in time relative to a reference time, and sets a cardiac period, which is set according to a second characteristic waveform succeeding the first characteristic waveform, and the first characteristic waveform; wherein the measurement unit measures the cardiac measurement values on the basis of the tomography image, the characteristic waveform, and the cardiac period.