Under the Enforcement Rules of the National Health Promotion Act, smoking areas in coffee shops in Korea should be divided off from other areas. The effect on indoor air quality of different division types for smoking areas was evaluated. Using real-time monitors, fine particulate matter <2.5 ${mu}m$ in diameter ($PM_{2.5}$) concentrations were measured simultaneously in the smoking and non-smoking areas of 30 coffee shops in Seoul. Average $PM_{2.5}$ concentrations in smoking and non-smoking areas were 132 ${mu}g/m^3$ and 52 ${mu}g/m^3$, respectively; significantly different. Average $PM_{2.5}$ concentrations in non-smoking areas were 39 ${mu}g/m^3$ in the glass-wall type and 64 ${mu}g/m^3$ in the separate-floor type. These $PM_{2.5}$ levels were above the US national ambient air quality standard of 35 ${mu}g/m^3$. Although indoor $PM_{2.5}$ levels in non-smoking areas were reduced by the division, the rates of reduction were not significantly different by division type. Our results demonstrated that $PM_{2.5}$ from smoking areas can infiltrate into non-smoking areas. Therefore, a complete indoor smoking ban in coffee shops is the only way to protect customers and workers in non-smoking areas.