Objectives: This study was conducted in order to evaluate $PM_{2.5}$ concentrations at 20 offices connected to the manufacturing industry from the beginning of September to the end of November 2012. Methods: A total of 20 samples were collected from 20 office buildings. Each $PM_{2.5}$ sample was collected by a 37 mm PTFE filter attached to a Personal Environment Monitor. Results: The geometric mean concentrations of $PM_{2.5}$ in the offices was $23.47{mu}g/m^3$, and the mean $PM_{2.5}$ concentrations measured in smoking offices were much higher than those of measured in non-smoking offices($24.83{mu}g/m^3$ and $21.55{mu}g/m^3$, respectively). $PM_{2.5}$ was revealed to be higher in small offices($39.52{mu}g/m^3$) than in medium or large offices($22.69{mu}g/m^3$ and $11.04{mu}g/m^3$, respectively). The mean $PM_{2.5}$ concentration of offices located on the 1st floor was higher than that of those on the 2nd floor, and those of offices located in the workplace were higher than those out of the workplace. The multiple regression model showed that concentration of $PM_{2.5}$ was positively associated with the method of ventilation. Conclusions: Smoking, ventilation method, location, and inflow of outdoor particulate matter are the most important factors for office $PM_{2.5}$ concentrations.