A replenisher which is capable of supplying Zr ions to a metal surface treatment solution, while suppressing an increase in the HF concentration in the metal surface treatment solution, so that a chemical conversion coating film can be continuously formed on a steel sheet by electrolysis and contains (A) zirconium hydrofluoric acid or a salt thereof and/or (B) hydrofluoric acid or a salt thereof and (C) a fluorine-free zirconium compound. The total concentration (g/l) of zirconium ions derived from the components (A) and (C) is 20 or more, and the ratio of the total molar amount (MF) of the fluorine ions derived from the components (A) and (B) relative to the total molar amount (MZr) of the zirconium ions derived from the components (A) and (C), namely MF/MZr is 0.01 or more but less than 4.00.