An active type isothermal calorimeter which can trace the progress of a liquid phase reversible reac-tion was constructed using electronic circuits and a digital computer. Heat storage capacity, heat of reaction, reaction equilibrium constant and reaction rate constants of a Diels-Alder reaction between maleic anhydride and 2-methyl furan in dioxane were estimated from the response curves of the calorimeter. The heat storage capacity of this mixture showed the maximum value of 6.89J/cm³K and 7.0mol/l initial concentration of maleic anhydride.