To numerically evaluate a residual treatment capacity of an ion exchange demineralizer.

A total treatment capacity value AALLof the demineralizer is calculated from total pure water collection amount (standard total pure water collection amount) in a demineralizer to be evaluated when standard water having a prescribed electric conductivity is used as water to be treated, as AALL=(standard total pure water collection amount)×(the electric conductivity of the standard water). The passage amount L of raw water through the demineralizer, and the electric conductivity S of the raw water at the primary side of the demineralizer are measured. The treatment capacity reduction ALOSSdue to the passage of the raw water is calculated from the passage amount L and the electric conductivity S as ALOSS=S×L. The residual treatment capacity value ARESTof the demineralizer is calculated as AREST=AALL-ALOSS.

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