By monitoring the concentration of NO gas in exhaled breath, the degree of inflammation of the airway due to asthma or the like can be evaluated. We have been developing an optical sensor to monitor NO gas in exhaled breath using Cobalt(II) tetraphenylporphyrin (CoTPP) as a sensing material. In this article, I introduce our efforts to increase the sensitivity of the sensors and to suppress influence of the atmospheric humidity on their characteristics, which are necessary for manufacturing practical sensors. CoTPP was dispersed in three kinds of hydrophobic polymers: ethyl cellulose (EC), polystyrene (PSt), and polycyclohexyl methacrylate (PCHMA). Due to the high dispersion of CoTPP molecules in the composite film, the sensitivity of the CoTPP/EC sensor was six times that of the film where only CoTPP solution was cast. The calculated NO gas detection limit for the CoTPP/EC sensor was 33 ppb. We also succeeded in suppressing the influence of humidity by making the film hydrophobic.