As a mass of forests planted after the second World War has grown to maturity, the harvestable volume of planted forests has increased rapidly in recent years in Japan. This study aimed at evaluating the effects of the maturity in Sugi (Cryptomeria japonica) planted forests on stumpage market. I built up a demand and supply equations system of Sugi stumpage market. The stumpage supply function included the variable, mature forest area, which is defined as the area of planted forests with age classes of VIII and above. I estimated the equation system with the national time series data of 30 years from 1971 to 2000. Estimation results were generally good, and it was confirmed that the resource maturity affects positively to stumpage supply. I then measured from the estimation results the effects of exogenous variables on endogenous variables in each decades. It showed that the rapid increase of mature resources in the 1980s and 1990s was very influential as a factor that shifts the supply curve to the right and had relatively large impacts on the drop of price and the stability of quantity in the Sugi stumpage market in these periods.