Day-care center is one of living micro-environments for children. In urban area, day-care centers may be influenced by air pollutants emitted from the vehicle exhaust. In this work, diurnal variation of major pollutants and effect of outdoor air on indoor air quality were investigated using real-time instruments for a day-care center located near the heavy road. 48-h continuous monitoring at both indoor and outdoor were made in summer. The daycare center equipped with ceiling system air-conditioners was operated from 7:30 a.m. to 19:30 p.m. Indoor CO2 concentration responded greatly to the human activity. Indoor NO2 concentration shows a big difference between daytime and nighttime, implying that outdoor NO2 may penetrate into the indoor through opening of doors or windows during the daytime. Indoor to outdoor concentration ratio of submicron particle surface area is < 1 due to the penetration of outdoor ultrafine particles. Keywords : Day-care center, Indoor air quality, NO2, Particle, Roadside