Engels noticed that the scientific facts, acquired in every field of 19th century natural science, were understood idealistically notwithstanding that they implied "materiality of world." He observed this point especially in thermodynamics in connection with the law of conservation of energy. That is, notwithstanding that the law of conservation of energy implied the integral conception of nature, it was understood as a scientific fact restricted to the field of thermodynamics because of the limits of Newtonian mechanical "Force" and method. And he thought that scientists employed "Force" as a common name which indicated the causes of phenomena unproved yet, and moreover their "Force" was only a fiction separated from matter. Therefore he asserted that Newtonian mechanical method was not suitable for explanation of nature in flux, because it was restricted to the fixed categories. Engels proposed "matter-in-motion" (or "Motion") as another concept that was capable of surmounting the limit of concept of "Force" and giving the whole description of nature. In this thesis I investigated that Engels recapitulated the law of conservation of energy by applying the concept of "Motion", after he recognized that the law of conservation of energy implied the integral conception of nature. Engels' recapitulation is as follows. The various forms of motion in nature are external manifestation of "Universal Motion" which exists in nature. And Universal Motion is constant notwithstanding that the various forms of motion in nature are transformed mutually.