To develop an NO 2 sensor that can measure the NO 2 concentration in the atmosphere, we studied changes in fluorescence intensities of dyes impregnated in porous glass when exposed to NO 2 in N 2 as the first step. The NO 2 sensing materials were Squarylium dye 1, Squarylium dye 2, and Rhodamine B. We found that fluorescence quenching by NO 2 occurred even in porous glass. The fluorescence quenching (F 0 /F) increased as the NO 2 concentration increased in the range of 0.2-12.3 ppm, with an almost linear relationship. The minimum detectable concentration of NO 2 was 0.2 ppm.