A powder stirring device includes a reaction container and a rotation driving device. The reaction container has a cylindrical outer peripheral wall and a pair of end surface walls. The end surface walls are respectively provided at one end and the other end of the outer peripheral wall. The reaction container is arranged in a heat-insulating cover such that an axis of the outer peripheral wall is in parallel with a horizontal direction. The outer peripheral wall has an inner peripheral surface rotationally symmetric with respect to the axis. During the powder process, the powder is stored in the reaction container, and the reaction container is rotated around a rotation axis that passes through the central axis of the outer peripheral wall by the rotation driving device. In this state, a processing gas is supplied into the reaction container. Also, the processing gas in the reaction container is discharged.