PURPOSE: To provide a kiln in which heat transfer rate is improved as compared with well known kilns while reducing loss of solid substances being carried away on a gas flow.

CONSTITUTION: A kiln 11 is provided, at least on a part of the inner circumferential wall thereof, with a plurality of protrusions 2 and arranged to enhance calcination efficiency by improving contact of solid substances to be calcinated and a gas flow. The protrusions 2 are shaped so that the powder is stirred but not lifted substantially and prevented from being carried away on a gas flow. Preferably, the protrusion 2 has the shape of a triangular prism wherein the top edge of the prism is the front edge merging from the powder bed through rotation of the kiln 11 and no powder is placed on the upper surface, i.e., the face 4 of the triangle.