Embodiments relate to a polymerizable composition comprising a phosphorus-based mold releasing agent for an optical use whose preparation process is improved, and a process for preparing the phosphorus-based mold releasing agent. The phosphorus-based mold releasing agents can be prepared more easily and conveniently by using phosphorous pentoxide, which can easily react with a monoalcohol and/or water at room temperature in the absence of a catalyst or a solvent. In addition, byproducts are not generated during the reaction. Thus, when a lens is produced by using the mold release agent obtained therefrom, it is possible to prevent a defective appearance of the lens that may be caused by byproducts, thereby further enhancing the appearance properties thereof. Further, since separate steps for removing byproducts, specifically, such steps as washing and filtration, are not required, wastewater is not generated. Thus, it is not only environmentally friendly, but the production cost can also be reduced as the process is simple.