Styrenated antibody (ST-Ab) and styrenated gelatin (ST-gelatin) were prepared by condensation reaction of antibody or gelatin with 4-vinylbenzoic acid, respectively. The affinity loss of ST-Ab to its antigen was minimal. ST-Ab and ST-gelatin were copolymerized with by visible-light irradiation in the presence of a water-soluble camphorquinone as a photoinitiator to produce a tissue-adhesive, in situ-formed co-gel of ST-gelatin and ST-Ab. The amount of non-reacted ST-Ab released from the co-gel of ST-gelatin and ST-Ab into the medium was minimal. The confocal laser scanning microscopy observation showed that local accumulation of rhodamine-labeled bovine serum albumin (BSA) as a model antigen was noticed in the surface-to-subsurface region of the co-gel of ST-gelatin and anti-BSA ST-Ab, indicating that the gel prevented the permeation of BSA into the gel. In invasion double chamber assay using anti-hepatocyte growth factor (HGF) antibody, the co-gel prevented HGF-dependent invasion of pancreatic cancer cells. The discussion was made for potential application of an in situ-formed tissue-adhesive co-gel of ST-gelatin and ST-Ab, developed in this study, as a cytokine-barrier on a surgically resected tissue where cancer cells might still remain after resection of cancerous tissue.