Background: Indoor PM2.5 concentrations in residential houses can be affected by various factors depending on the season. This is because not only do the climate characteristics depend on the season, but the activity patterns of occupants are also different. Objectives: The purpose of this study is to compare factors affecting indoor PM2.5 concentrations in apartments and detached houses in Daegu according to seasonal changes. Methods: This study included 20 households in Daegu, South Korea. The study was conducted during the summer (from July 10 to August 10, 2023) and the autumn (from September 11 to October 9, 2023). A sensorbased instrument for PM2.5 levels was installed in the living room of each residence, and measurements were taken continuously for 24 hours at intervals of one minute during the measurement period. Based on the air quality monitoring system data in Daegu, outdoor PM2.5 concentrations were estimated using ordinary kriging (OK) in Python. In addition, the indoor activities of the occupants were investigated using a timeactivity pattern diary. The affecting factors of indoor PM2.5 concentration were analyzed using multiple regression analysis. Results: Indoor and outdoor PM2.5 concentrations of the residences during summer were 15.27±11.09 μg/m3 and 11.52±7.56 μg/m3, respectively. Indoor and outdoor PM2.5 concentrations during autumn were 13.82±9.61 μg/m3 and 9.57±5.50 μg/m3, respectively. The PM2.5 concentrations were higher in summer compared to autumn both indoors and outdoors. The primary factor affecting indoor PM2.5 concentration in summer was occupant activity. On the other hand, during the autumn season, the primary affecting factor was outdoor PM2.5 concentration. Conclusions: Indoor PM2.5 concentration in residential houses is affected by occupant activity such as the inflow of outdoor PM2.5 concentration, cooking, and cleaning, as found in previous studies. However, it was revealed that there were differences depending on the season.