PURPOSE: To prevent clogging of pipelines, etc. during shutdown by providing a storage tank for purging liquid, and connecting a feed pipeline for mixed fluid and said tank and connecting a producing device for mixed fluid and said tank.

CONSTITUTION: A storage tank 26 for purging liquid is separately provided and said tank 26 and a feed pipeline 18 for mixed fluid are connected by an inflow pipeline 24 for purging liquid. A producing device 10 for solid-liquid mixed fluid and the tank 26 are connected by a circulating pipeline 29. During the shutdown of the device, the mixed fluid in the pipeline 18 is purged into the tank 26 and in the stage of resuming the operation, the liquid in the tank 26 is fluidized cyclically between the tank 26 and the device 10 and during this time solids are supplied to the device 10 so that the circulating liquid forms the mixed fluid of prescribed mixing ratios.