PURPOSE: To permit efficient microwave drying of granular body to be treated which is provided with a small microwave loss coefficient by a method wherein a mixer, consisting of microwave absorbing material and mixing the granular body to be treated, is provided in a vessel receiving the granular body to be treated.

CONSTITUTION: A mixer or vanes 17, for example, which mixes granular body 10 to be treated, is provided in a vessel 18 while the vanes 17 are constituted of a microwave absorbing material or a material having a large microwave loss coefficient such as the ceramics of SiC, SiN, ferrite material for example. When an electric power source is impressed on a microwave oscillating source 1 to oscillate microwaves and the microwaves are sent into an oven 7 through a waveguide 2 to project them against the granular bodies 10 to be treated and the mixer or the vanes 17, the vanes 17 absorb the microwaves and generate heat. At the same time, the vanes 17 generating heat are driven by a motor 20 to rotate and mix the granular body 10 and, therefore the generated heat is transferred to the granular body 10 and/or contacted directly with the granular body 10 to be treated.