Burning, everyday, several incense sticks through a year, the author has found that they burn more rapidly in summer than in winter, whilst the daily variation of their burning velocity is subjected to the changing relative humidity of surrounding air. Further, using several other materials the influence of humidity on the burning together with house-fires is thoroughly investigated, thus: 1. The most important factor of the problem among the numerous meteorological elements is the relative humidity. 2. The combustion of some substances is influenced, in large degree, by the variation of the water-quantity within, when they burn in low temperature without flame. 3. The combustion of some substances is influenced by the water-quantity of air, when they burn with flame and temperature is moderately high. 4. The combustion of other substances is, if temperature is enormously high, controlled by the humidity of air, but in the way contrary to the proceeding, that is, they burn strongly with increasing humidity. 5. The influence of wind on fires is not so remarkable as it is now believed. 6. The fire-statistics in many cities and prefectures in Japan indicates that the outbreak of fires has the most intimate correlation with the relative humidity among many other meteorological elements. 7. The outbreak of fires undergoes a change yearly and daily. It corresponds in many respects with the seasonal and daily variations of the moisture in timber, paper and cloth etc. in the room. Therefore we can conclude that the relative humidity has the great influence not only on the burning but also on the fires.