An object is to suppress a significant change in electrical characteristics of thin film transistors and a deviation thereof from the designed range due to static electricity, and to improve the yield in manufacturing semiconductor devices. In order to prevent a substrate from being charged with static electricity by heat treatment or to favorably reduce static electricity with which a substrate is charged in a manufacturing process of a semiconductor device, heat treatment is performed with a substrate provided with a thin film transistor stored in a conductive container. In addition, a heating apparatus for performing the heat treatment is electrically connected to a ground potential, and the container and the substrate are also electrically connected to the ground potential.