Clothing systems for 16 infants aged from 0.5 to 1 year-old were investigated through a survey of changes in clothing conditions according to season and a series of experiments using a baby thermal manikin. Based on the results, thermal resistance of an infant's clothing ensemble can be estimated by a derived equation employing its total weight. The thermal resistance of the ensemble depends upon the season. The results obtained were 0.53-0.81 clo in summer, 0.42-1.33 clo in autumn, and 1.03-1.90 clo in winter. In order to maintain thermal equilibrium in a resting clothed infant, favourable thermal resistance for the ensemble should be less than 0.3 clo in summer, less than 0.5 clo in autumn, and 0.8-1.8 clo in winter. Therefore, it may be said that the infants wore more or less appropriate clothing ensembles in winter, while the thermal resistance of their ensembles in summer and autumn was somewhat excessive.