A neuronavigation system indicates a lesion area of a patient on preoperative medical images of the patient, and hence such medical navigations have been widely used in neurosurgeries. However, the brain deformation called brain shift causes inaccurate navigation. This paper presents a method of brain shift estimation and a way to show the deformed brain to surgeons using Augmented Reality (AR) technology. By acquiring posture information of the brain and coordinating transformation, the direction of gravitational force is determined. Infrared reflective markers are attached to a camera and human head, and the markers are tracked by a motion capture system. A computer graphics (CG) model of the deformed brain is superimposed onto the camera image.