An ultrasound diagnosis apparatus according to an embodiment includes processing circuitry. The processing circuitry obtains time-series data having complex values based on a reflected wave of an ultrasound wave transmitted by an ultrasound probe and calculates an expansion coefficient in a case in which the obtained time-series data is expressed as a linear sum of a plurality of mathematical functions, the time-series data having, as an argument, a first parameter related to time. The plurality of mathematical functions are mathematical functions that are possible to be generated on a basis of a function family that has, as arguments, the first parameter and a second parameter different from the first parameter.
