The paper describes in situ observation of the photopattern formation in polymer Langmuir-Blodgett (LB) film by surface plasmon spectroscopy. The copolymers (p(DDA/tBVPC)) of N-dodecylacrylamide (DDA) which is known to form a stable monolayer with a photo-degradable monomer, tert-butyl-4-vinylphenyl carbonate (tBVPC), form a stable monolayer and LB film. When the copolymer LB films were irradiated by deep UV light in air, the photodecomposition took place, consequently the thickness in the LB film was gradually decreased. On the other hand, there were no significant changes in the thickness with the irradiation at the argon atmosphere. It was found that the molecular oxygen plays a decisive role in yielding the positive-tone pattern in the p(DDA/tBVPC) LB films.