The present invention provides an elastase-inhibiting protein isolated ("Guamerin") from a Korean leech, Guameri (Hirudo nipponia) and a process for preparing the same. Guamerin is a protein of a molecular weight of 6,110 Da which is composed of 57 amino acid residues, whose active site is composed of 36-methionine and 37-isoleucine, which retains an inhibiting-activity highly specific to elastase, and which shows stability against heat as well as strong acids and alkalies. Guamerin of the present invention can be applied in the treatment of diseases associated with an excess level of elastase, such as rheumatoid arthritis, emphysema, and psoriasis.