Abstract: Minimally Invasive Surgery (MIS) is usually performed in a needle insertion or a laparoscopic surgery. To minimize an incision, Remote Center of Motion (RCM) has been applied to surgery. A surgical manipulator has to be placed exactly on the RCM point during the surgery. However, a constraint force to maintain the placement exists. It may affect a patient's skin, so the constraint force should be controlled to be zero. In order to derive the constraint force model, a simple serial manipulator is taken as an exemplary device. The kinematics is derived first. The serial manipulator fixed to the RCM point can be considered as a five-bar closed chain. To derive the constraint force model, an open-chain dynamics is obtained first, and then, the kinematic constraints is applied. As a result, the constraint force model can be derived. Finally, the constraint force is confirmed through simulations.