Utilizing the existence of a sufficiently long induction period during photo-polymerization, defined as the time required to initiate macroscopic gelation after UV irradiation, we propose a new injection method of making a photo-polymerized hydrogel made of thermo-sensitive di-acrylated Pluronic F 127 (DA-PF 127). First, the photo-polymerization of DA-PF 127 solution at the molecular level is initiated by UV irradiation, and this solution is injected into a target site by macroscopic gelation before it becomes viscous. This method can overcome the problems of the existing methods to make an injectable and stable hydrogel by photo-polymerization, reducing the potential damage to normal tissue around the injection site due to direct UV exposure, and the requirement of special equipment for UV crosslinking after injection. By controlling photo-polymerization variables, we found the condition for making an injectable system, where the induction time is equal to or longer than the UV irradiation time. The feasibility of the proposed method was demonstrated in vitro, and the enhanced stability of the produced hydrogels by photo-polymerization was verified. We also characterized the cytotoxicity of the present method using cell cultures and cell encapsulation with the present method, and found minimal cytotoxicity.