Engineered wood materials, such as the urea-formaldehyde resin adhesives used to make plywood, are known to be one of the primary sources of formaldehyde emissions in the building industry. This study presents the results of measuring indoor formaldehyde concentrations in newly built apartment buildings at the pre-occupancy stage for more than three years. Formaldehyde emissions from engineered wood materials used in floorings and furniture were measured using the small chamber method. Indoor formaldehyde concentrations were measured by following, for the most part, the procedure proposed by the Korean Ministry of Environment. These measurements verified that improving the engineered wood materials used in the apartment buildings had an effect on lowering the formaldehyde concentrations. The results showed apparent decrease in formaldehyde concentrations when low-emission engineered wood materials were used. Regardless of the absolute decrease, seasonal fluctuations in formaldehyde concentrations were also observed. It was assumed that these fluctuations were caused by changes in the indoor air temperature and relative humidity. Nevertheless, it will be necessary to conduct further studies to identify the factors causing the weak correlation of formaldehyde concentrations with the indoor air temperature and relative humidity in each housing unit.