PURPOSE: To prevent the re-adsorption of the ion of an alkali metal to the surface of a semiconductor substrate after the alkali metal is separated from the surface by ionization by cleaning the semiconductor substrate with a chemical prepared by mixing a metal which is adsorbed to the semiconductor substrate more strongly than the alkali metal in pure water.

CONSTITUTION: An Si substrate 8 is dipped in a mixed solution of NH4OH, H2O2, and H2O contained in a chemical bath 2. Then the substrate 8 is moved to a rinsing bath 3 and dipped in a rinsing solution prepared by mixing Al in pure water at a concentration of ≤1ppb. In case the substrate 8 adsorbs an alkali metal, the metal is separated from the substrate 8 by ionigation in the rinsing liquid in the bath 3 and, at the same time, the Al contained in the rinsing solution is adsorbed to the surface of the substrate 4 prior to the alkali metal. The alkali metal is not adsorbed to the substrate 4, but is made to overflow the edge of the bath 3.