To enable a large quantity of excrementitious matter to be stored without giving out an offensive smell by mixing an excretian treating agent containing quicklime as a principal component with excreta.

When one relieves oneself in a simple toilet, excreta fall down through a toilet bowl; however, at that time a second shutter 30 at the underside of a mixing part 12 is closed, so that the excreta are stored in the mixing part 12. Then, a finish switch equivalent to the flush in an ordinary toilet is turned on, and a mixing device 14 starts to operate, and at the same time a first shutter 26 is opened, and a given quantity of an excretion treating agent 20 in a treating agent housing container 22 is discharged into the mixing part 12 through a supply pipe 24. This excretion treating agent 20 is mixed with the excreta by the mixing device 14, and the quicklime in the excretion treating agent 20 reacts with the water content in the excreta to become slanked lime, and also emits heat. Then, a part of water content in the excreta is absorbed in the slaked lime by this reaction, and the other water content is evaporated by the heat at the time of the reaction, so that the excreta in a muddy shape is converted into a dried granular shape.