To provide a method capable of effectively dissolving waste paper including foreign materials and inexpensively carrying out the method with a saving space.

This method is to collect almost all of foreign materials R1, R2 brought to a screening vessel 2 with dissolved old paper in the screening vessel 2, remove them from here, collect the foreign materials R2 heavier than water on the bottom area of the screening vessel 2 and discharge them from the bottom area by a specific weight part-removing device 6.