<p>　Activity-based working (ABW) is a new working style that aims to improve the workplace productivity of office workers. In ABW offices, the work engagement of office workers is expected to improve as workers shift to a working style that allows them to think and act for themselves. The spread of COVID-19 has led to an increase in new ways of working that combine office work and telework. Recently, the introduction of ABW in a huge single-floor space, called a mega plate office, has attracted attention as a way to further increase the effectiveness of ABW. Few studies have been conducted on workplace productivity in mega plate offices with ABW. However, to the best of our knowledge, no case study has been conducted that directly evaluates the relationship between work engagement and workplace productivity in mega plate offices. Therefore, this study aims to evaluate the impact of the indoor environment of a mega plate office on the workplace productivity of workers, the relationship between work engagement and workplace productivity in mega plate offices, and the impact of mega plate offices on the workplace productivity of workers in a working style that combines both office work and telework. </p><p>　Indoor environmental measurements and questionnaires were conducted in a previous office, a mega plate office winter with a traditional work style, and a mega plate office summer with a work style that combines office work and telework. </p><p>　The percentage within the thermally acceptable range was higher in the mega plate office summer than in the previous office. The average CO2 concentration was low in the mega plate office winter. In addition, the average CO2 concentration was even lower in the mega plate office summer because the presence rate was 25%. The perimeter of the mega plate office had high illuminance both during winter and summer due to daylight from a large opening on the east side. </p><p>　Most workers responded more satisfaction to the temperature, indoor air quality, and space environment in the mega plate office winter than in the previous office. In addition, most workers in the mega plate office summer responded more satisfaction to the indoor environment. In the mega plate office winter, the number of workers who selected the indoor environment as the reason for seat selection decreased; further, several workers selected a seat based on communication and work content. Further, in the mega plate office summer, the number of workers who selected communication as the reason for seat selection increased because they came to the office when face-to-face communication was necessary. In the mega plate office winter, the meeting space was expanded, making communication easy. Moreover, the addition of a refreshment space facilitated refreshing breaks. In the mega plate office summer, the number of office workers who selected communication as the reason for seat selection increased, making communication easy. </p><p>　In the mega plate office winter, workers with high work engagement could fully exercise their ability to select their work environment because of the high selectivity of the work environment; further, they could concentrate on their work and refresh themselves easily. The workers with higher work engagement were more likely to engage in creative activities because the presence rate decreased in the mega plate office summer, further increasing the selectivity of the work environment. </p>