PURPOSE: To provide a hydrogen ion type strongly acidic cation exchange resin without any leak of impurity cations.

CONSTITUTION: The hydrogen ion type strongly acidic cation exchange resin has, on the basis of the total exchange equivalent of the ion exchange resin, 0.1 equivalent % or less impurity cations other than hydrogen ions. The method for preparing the hydrogen ion type strongly acidic cation exchange resin comprises 1) a step to prepare a crosslinked polymer, 2) a step to sulfonate the crosslinked polymer in the presence of a sulfonating reagent, and 3) a step to hydrate with water exhibiting an electric resistance of 5 MΩ cm or higher at 25°C, or with diluted sulfuric acid and subsequently with water exhibiting an electric resistance of 5 MΩ cm or higher at 25°C.