Urban and forest areas are neighboring in Japan and people use the forest area variously and frequently. Thus people's carelessness is one of the major reasons for forest fire. The hazard map drawing and people control for law hazard area are thought to be the effective countermeasure. The model was constructed to estimate the water content ratio as the index of forest fire based on the micro weather data in the forest. Litter on the forest floor is compared to a bucket in this model and water budget is calculated based on the precipitation and evaporation. The necessity data are solar radiation and precipitation. This model has 4 constant parameters. 3 of them are for the relationship between evaporation and solar radiation. Another parameter is the maximum water content ratio of the litter. 2 adjoining plots were settled in the deciduous forest that is the forest type in the highest incidence of forest fire in Japan. The litter in one plot did not get a lot of sunshine and tended to be wet caused by the evergreen trees in high density. Water content ratio of the litter and solar radiation on the forest floor were measured in each plots for one year. Parameters were fixed with the measured data. The variation of the water content ratio was estimated well except for the extreme high ratio just after the precipitation events. However, the parameter values were much different between 2 plots.