Abstract: A method for expressing, as a soluble protein or an active-form mutant enzyme, an enzyme that cannot be expressed as a soluble protein or an active-form enzyme in a heterologous expression system or that is obtained in a minute amount even when an active-form enzyme is expressed, the method including a technique for selecting an effective mutation site and a mutated amino acid. A new active-form mutant enzyme is also disclosed. The method involves: specifying an insoluble protein or an inactive-form enzyme; specifying a hydrophilic amino acid in a hydrophobic domain and/or a hydrophobic amino acid in a hydrophilic domain of an ?-helix structure portion of the insoluble protein or the inactive-form enzyme and preparing a gene that codes for an amino acid sequence in which a substitution is made to the hydrophilic amino acid in the hydrophobic domain and/or the hydrophobic amino acid in the hydrophilic domain.