The focused ultrasound wave attracts attention in the medical field in various applications such as High Intensity Focused Ultrasound (HIFU). Amount of sound energy is generated by focused ultrasound in the body of narrow area. Present HIFU treatment cannot be applied to the part surrounded by bones, such as brain, because focal point is changed by refraction and reflection of ultrasound. In this research transskull ultrasound propagation is analyzed by finite difference method for HIFU treatment to brain. Assumption of isentropy and small disturbance of Euler equation gives the governing equation. The density and speed of sound of each medium are given at the center of Cartesian grid cells so as to model arbitrary human body from Computerized Tomography scan images. As wavelength of an ultrasound is very short, large amount of grid points is required. Therefore, parallel computing using MPI is performed.