In this study, we developed the measuring system based on light addressable potentiometric sensor for the quantitative analysis of penicillin that is very important element in medicine and pharmacy, clinic. It is investigated the response characteristics by enzyme reaction with penicillinase. First, the surface pre-treatment process of the Si₃N₄ was established. The coupling agent was made using self assembled monolayer method and it was confirmed the immobilization process by AFM. Also, as the measuring system, potentiostat, signal processing part etc. was made by Lab VIEW software; it was reduced detecting time as well as simplifying the system. Fabricated device was shown excellent pH response characteristics, 57 mV/ pH in the range of pH 2~11. The response characteristics was 60 mV/decade in the range of 0.1~10 mM.