To rapidly dissolve plural pieces of a solid treating agent into water to dilute the agent and to facilitate the preparation of a treating solution by providing the device with a solid treating agent trapping means placed inside a solid treating agent dissolution vessel and a solid treating agent side circulation/delivery means having a solution delivery section placed on the solid treating agent side with respect to the solid treating agent trapping means.

This device is provided with: a circulation means 2 that consists of a circulation pump 4, a solid treating agent side circulation/delivery means 5 and a circulation/delivery means 6 and is used for circulating a treating agent solution through a suction port 3 inside a solid treating agent dissolution vessel 1 and a water transfer pipe, wherein the suction port 3 is fitted with a filter or the like; and a solid treating agent trapping means 8 that is placed inside the solid treating agent dissolution vessel 1 and is used for preventing a solid treating agent 9 from being deposited on the inside of the solid treating agent dissolution vessel 1 or from blocking the liquid flow passage extending to the circulation means 2. At this time, in order to apply water to a broad region of the surface of each of the pieces of the solid treating agent, a part of the shape of the solid treating agent trapping means 8 is formed so as to be convex downward and also, the trapping means 8 is coated with a material having high slip properties, such as Teflon(R).