PQQGDH having an improved substrate specificity or having an improved specific activity in an assay system using ferricyanide ion as a mediator is provided. Modified PQQGDH having the enhanced substrate specificity by introducing an amino acid mutation in a particular region of PQQGDH, and a method of enhancing the specific activity compared with a wild type in the assay system using the ferricyanide ion as the mediator by deleting, substituting, or adding one or more amino acids in an amino acid sequence of the wild type pyrroloquinoline quinone dependent glucose dehydrogenase.