PURPOSE: To purify the titled compound effective for promoting insulin secretion industrially and advantageously, by bringing a fused protein obtained by bonding a peptide having a specific amino acid sequence to a protein having specific molecular weight in an clevable way into contact with an adsorbent of silica type and eluting.

CONSTITUTION: A gene to code a fused protein obtained by bonding spittle peptide P-C having amino acid sequence shown by the formula, found in the spittle, to a protein (e.g. β-D-galactosidase) having ≥10,000 in an clevable way is prepared. The gene is linked to a manifestation vector and inserted into Escherichia coli, which is transformed. Then the transformed Escherichia coli is cultivated to give a supernatant liquid of culture containing a large amount of a protein comprising a peptide having the amino acid sequence shown by the formula. Further the supernatant liquid is passed through a column packed with a silica-based adsorbent, the eluate is made to flow and the adsorbed substance is eluted to give the aimed fused protein.