To remove a residue to an amount far less than a conventional amount of the residue thereby to minimize loss in starting production by solving the problem that the residue such as ash remaining in an element comes into a molten resin at the time of passing the liquid again has to be tolerated as an inevitable and the duration until a foreign matter is exhausted is admitted as the loss in starting the production.

A method for cleaning a filter is to heat the element of the filter used for removing the foreign matter in a resin under an atmosphere of high temperature steam, decompose an organic matter and remove. Thus, the amount of the residue can be reduced.