The present invention provides a novel Kunitz-type inhibitor protein exhibiting high potency to inhibit serine protease activity of human blood coagulation factor Xa. The Kunitz-type inhibitor protein against human blood coagulation factor Xa according to the present invention isa Kunitz-type inhibitor protein: bitoran D from Bitis arietans venom having the following amino-acid sequence I:;
(SEQ ID NO:1)SKKRP DFCYL PADDG PCRAF IPSFY YNSTS NECNT FIYGGCYGNA NKFES MDECR KTCVA SA,
a Kunitz-type inhibitor protein: bitoran V from Bitis arietans venom having the following amino-acid sequence II: ;
(SEQ ID NO:2)C RQNRP DFCYL PAVEG PCRAY IRSFF YNSTS NECEK FFYGGCYGNA NKFET RDECR KTCVA SA,