A laser diode with which high density crystal defect and surface roughness are able to be inhibited from being generated is provided. Accordingly, Aeromonas aminopeptidase can be used to remove N-terminal alanyl residues from derivatives of human somatotropin, porcine somatotropin, and bovine somatotropin, for example, to yield proteins having their native amino acid sequences. The ASHRAE or HEPA filter element is relatively inexpensive and easy to clean.