We propose a new weight compensation mechanism with a non-circular pulley and a spring. We show the basic principle and numerical design method to derive the shape of the non-circular pulley. After demonstration of the weight compensation for an inverted/ordinary pendulum system, we extend the same mechanism to a parallel four-bar linkage system, analyzing the required torques with transposed Jacobian matrices. Finally, we develop a 3 D.O.F manipulator with relatively small output actuators and verify that the weight compensation mechanism greatly contributes to decrease static torques to keep the same posture within manipulator's work space.