A rotary kiln includes: a cylindrical shell that includes a supply-side end portion and a discharge-side end portion at both ends in an axial direction of the shell, and a heat treatment chamber which is defined inside the shell and in which a heat treatment is performed on a process material; a supply-side holder that holds the supply-side end portion; a discharge-side holder that holds the discharge-side end portion; a supply-side rotary shaft that allows rotation of the supply-side holder; and a discharge-side rotary shaft that allows rotation of the discharge-side holder. The shell is rotated about its own axis by rotating at least one of the supply-side rotary shaft and the discharge-side rotary shaft.