A mutant fungus strain which produces a protease with low thermostability and low productivity was made from protease producing fungi and the gene coding for mutant enzyme is isolated from the mutant strain. A promoter which can function in yeast is ligated to the gene and inserted into a plasmid replicable in yeast, and the resulting plasmid is introduced into yeast. The yeast is cultured, thereby producing the mutant enzyme. Furthermore, a gene expressing an enzyme with a far lower thermostability is prepared by site-directed mutagenesis, which is introduced in yeast, thereby producing an enzyme with more distinctively reduced thermostability.