Cleaning device for inside of tube

A large number of small diameter tubes are disposed passing through a lower end plate of a main body of a reactor pressure vessel. The device of the present invention can wash the inside of the small diameter tube efficiently and forecast the completion time of the operation. Namely, purified water at a predetermined temperature is stored in a water reservoir of a purified water recycling unit. The purified water is introduced from a header to the inside of the tube by way of a short tube connected to the lower end of the tube. The purified water introduced into the tube is charged from an opening to an overflow tube and returned to the water reservoir passing through the over flowing tube. The inside of the tube is washed by such circulation of purified water. In the cleaning operation by using the device, (1) the completion time of the operation can be forecast and operationability is improved, and (2) since the operation can be performed rapidly, the period of time for performing the step can be shortened. (I.S.)