Ultrasound data of a biological tissue is acquired in a relaxed state and a tensed state. A strain is estimated directly from the relaxed state ultrasound data and the tensed state ultrasound data by way of an imaginary part of a complex correlation function. In estimating the strain, the tensed state ultrasound data is re-stretched, and the imaginary part of the complex correlation function is calculated from this re-stretched ultrasound data and the relaxed state ultrasound data. A slope factor is also calculated, and the strain is estimated from this slope factor and the imaginary part of the complex correlation function.
