A possible mechanism for the formation of the substrate-induced interface during the gelation, which leads to heterogeneous polymerization, is proposed. According to this mechanism, a surface polymer deplete layer would be formed during the polymerization due to the high interfacial energy between the hydrophobic substrate and the aqueous polymerizing solution. When the polymer concentration attains the critical value at which an extensive entanglement occurs, and/or a chemical network forms, the thickness of the deplete layer increases dramatically to a macroscopic level to give an interface, due to the weak elasticity of the network. Theoretical model describing relations between the substrate surface tension, the increment in the surf ace tension of the solution upon polymerization, and the predicted position of the interface show a resonable agreement with the experimental data.