Because anti-Marxist’s criticism against Fundamental Marxian Theorem(FMT) is based on an assertion that this proof can be understood as a ‘sun-power exploitation,’ if we assume, for example, ‘Sun-power Theory of Value’, we should prove not only exploitation but also Labor Theory of Value itself. Therefore, this paper aims to prove Labor Theory of Value mathematically by focusing on the historically conditional proportionality between labor input and amount of products which is assumed in Labor Theory of Value. By this proof of the conditional proportionality, we show that marginalist principle does not disturb Labor Theory of Value in capitalism at all. Furthermore, marginalist principle is important to show the labor process as a subjective optimization process which is not by the sun but only by human beings. In this way, we use anti-Marxist’s marginal principle to object anti-Marxist criticism against Labor Theory of Value.