ect transistor(FET) type dissolved carbon dioxide(pCO₂) sensor with a double layer structure of hydrogel membrane and CO₂ gas permeable membrane was fabricated by utilizing a H^+ ion selective field effect transister(pH-ISFET) with Ag/AgCl reference electrode as a base chip. Formation of hydrogel membrane with photo-crosslinkable PVA-SbQ or PVP-PVAc/photosensitizer system was not suitable with the photolithographic process. Furthermore, hydrogel membrane on pH-ISFET base chip could be fabricated by photolithographic method with the aid of N,N,N`,N`-tetramethyl ethylenediamine(TED) as O₂ quencher without using polyester film as a O₂ blanket during UV irradiation process. Photosensitive urethane acrylate type oligomer was used as gas permeable membrane on top of hydrogel layer. The FET type pCO₂ sensor fabricated by photolithographic method showed good linearity (linear calibration curve) in the range of 10^(-3)∼10 mole/ℓ of dissolved CO₂ in aqueous solution with high sensitivity.