In order to deduce water content of PM2.5 in quasi-real-time, an observation system for hygroscopic characteristics of fine particles by using an optical particle counter (OPC) was developed. Measurements of volume concentrations (V0.3-2.0) from 0.3 to 2.0 μm in diameter to approximate PM2.5 in both dried and ambient (without dried) conditions were conducted at inland and coastal sites located in the south part of Kanto plain, Japan from December 2016. The relationship between relative humidity and hygroscopic change of V0.3-2.0 was analyzed by using hygroscopicity parameters (κv). The mean values of κv were 0.37 and 0.58 at the inland and coastal site, respectively. The mean κv was larger than the value calculated from chemical compositions of PM2.5 based on the simple mixing rule at the coastal site. This result suggests that mixing states and organic species also have important role to determine the hygroscopic behaviors of PM2.5. Since our system is easy to handle without frequent maintenance, it is suitable for simultaneous observation at multiple points for long term. These data will help to obtain representative hygroscopicity parameters of PM2.5 at various regions and seasons.