PURPOSE: To provide a perpetual-motion machine acting as an infinite power source free of pollution by controlling potential energy by the use of the reaction of a spring, and concurrently making the potential energy of a weight the motive power of a periodic engine.

CONSTITUTION: A device is set in a periodic engine by the falcrum 5 of a pendulum main body, an arm in an L shape is so set with a second step falcrum 6 provided at the tip end of the arm that a force point of the pendulum main body 1 can be easily changed, and a force point is so designed as to be changed in a mutiplex manner by letting a spring 3 be extended/contracted by the gravity of a weight 2. At this time, with the second step 8 kept closed by the reaction of the spring 3, when the weight 2 positioned at a third step 9 is moved to the position of a weight 10, the force point is thereby shifted to a first step 7 from the third step 9. Therefore, the use of potential energy by gravity as a power source for the periodic engine enables the device to be a perpetual- motion machine.