Most existing mechanical gravity compensators have been developed for revolute joints that are found in majority of articulated robot arms. However, robots such as patient transport robots use prismatic joints, which need to handle a heavy payload. In this study, a high-capacity linear gravity compensator (LGC), which comprises pure mechanical components, such as coil springs, a rack-pinion gear, a cam, and a wire, is proposed to compensate for the payload applied to a prismatic joint. The LGC is designed to generate a constant compensation force regardless of the payload position. The device can be manufactured at a low cost and has a significantly long lifespan because it uses coil springs to serve as an elastic body. Experiments demonstrate that the robot with the LGC can handle a load of 100 kg more than the robot using the same motors without it.