Neuronavigation has contributed to increase safety in neurosurgeries. However, the preoperative medical images used in the navigation become unreliable as a surgery progress because of deformations of the brain known as brain shift. Brain shift is partly caused by outflow of cerebro-spinal fluid and resultant decrease of buoyancy. In this paper, a simple estimation method for brain shift based on finite element method (FEM) and hydrostatics is proposed. The pressure of the fluid is calculated by hydrostatics and the deformation of the brain tissue is calculated by the FEM. Verification experiments were performed by using a block of gelatin, and the accuracy of the brain shift estimation method was verified.