A relation that represents "solid-liquid coexistence" curves in binary equilibrium phase diagrams of solvent crystal-noncrystalline polymer systems is deduced on a more general theoretical basis, which is a counterpart of the Flory-Huggins equation representing a solution curve. The relation deduced is expressed as a function of concentration in terms of the interfacial free energy and crystal size. Next, on this relation basis, a melting point depression analysis is made by using certain experimental results on lightly chemically cross-linked NR-benzene systems whose degree of chemical crosslinking is so small that they can be regarded as uncross-linked systems. The relation is revised when considering an elastic term by chain entanglements in the good solvent and a certain concentration-temperature dependent polymer-solvent interaction parameter. Consequently, in the higher concentration range, part of the previous data of crystal size determined from X-ray fit fairly well within the upper and lower limits of the data points calculated from the relation revised.