PROBLEM TO BE SOLVED: To provide an input amount determination method capable of reducing an input amount of unused fresh acid, in an acid cleaning treatment line using a used waste acid.SOLUTION: When an input amount of an acid cleaning liquid on an acid cleaning treatment line 1 using a used waste acid collected from an acid cleaning tank 11 and unused fresh acid, is determined, based on a descaling property and plate width of a hot rolled steel strip which is subjected to acid cleaning treatment, and an input state of the waste acid in the acid cleaning tank 11, an input amount of the fresh acid is determined.
