As one of surgical instrument, an abdominal retractor which has six-DoF with high reliability is developed as substitutes for use during surgery. The proposed retractor consists of a serial three-link arm and three ball joints. During surgery, the retractor needs to maintain the shape of the relevant body part while maintaining own posture. The proposed instrument can increase the efficiency of surgery. Spherical joints can be driven by operator manual operation through inner/outer cables. In particular, in order to strengthen the holding force, it is processing dimples on the ball. In addition, we introduce a knock mechanism as another holding/release mechanism.