This paper describes a strategy of null space force control of a redundant manipulator. As well known, robot is utilized in various industrial fields and it is preferable for the robot manipulator to have multiple degrees of freedom motion. Several studies of kinematics for null space motion have been proposed. However, the stability analysis of null space motion in force control is not enough. Then, passivity based null space force control is proposed. In this strategy, work space observer based control (position control) and null space control which can obtain desired force response are constructed. Moreover, its null space controller compensates stability of null space motion. Using L 2 disturbance suppression property of the system, an influence of force torque to each joints can be suppressed. Desired force response to the object is realized by force PID controller. The validity of the proposed approach is verified by experiments of 4-link redundant manipulator.