A process for producing a cross-linked crystallized protein complex, which comprises: a first step of concentrating a crude protein derived from an animal or plant; a second step of encapsulating the protein in a gel, to thereby allow the protein to undergo air oxidation, and then extracting a protein complex from the gel; a third step of allowing the extracted protein complex to undergo crystallization and precipitation; and a fourth step of cross-linking the precipitated protein complex. Alternatively, by use of a fifth step of drying (FD) the obtained crosslinked crystallized protein complex, to thereby form a powder. As a result, there is provided an enzyme which is stable at room temperature storage, and has an activity in catalyzing an asymmetric oxidation reaction. That is, there is provided a useful material which enables an efficient enzyme-mimetic reaction under a mild condition.