<p>Indoor environmental qualities in offices during the COVID-19 pandemic were analyzed on the following 4 aspects.</p><p>1. Indoor environments during the COVID-19 pandemic were measured in 20 office buildings, indicating that ventilation might have changed from pre-COVID-19.</p><p>2. A comparison of the indoor environment in 2020 and 2021 showed an increase in CO2 concentrations.</p><p>3. Occupancy rate and CO2 concentration had a positive correlation, indicating that workers’ return to offices in 2021 increased CO2 concentration.</p><p>4. The relationship between countermeasures for COVID-19 and indoor environments was analyzed, indicating that mechanical ventilation system might be effective during the COVID-19 pandemic.</p>