The present study investigated waste medium from a domestic shiitake mushroom farm, which was pyrolyzed to produce biochar. The yield rate of the biochar was compared after exposure to various pyrolysis temperature conditions, and the characteristics of the produced biochar were analyzed. The present study focused on the carbon dioxide (CO2) adsorption capacity of the resulting biochar. The CO2 adsorption capacity exhibited a correlation with the pyrolysis temperature of the biochar, with increasing temperatures resulting in higher CO2 adsorption capacities. Brunauer-Emmett-Teller (BET) analysis showed that the CO2 adsorption capacity was related to the surface area and pore volume of the biochar. Calcium is added to the process of producing mushroom medium. Experiments were performed to investigate the CO2 adsorption capacity of the biochar from the waste medium with the addition of calcium. In addition, CO2 adsorption experiments were conducted after the pyrolysis of kenaf biochar with the addition of calcium. The results of these experiments show that calcium affected the CO2 adsorption capacity.