602852 A method of producing soluble thrombomodulin with a content of host cell-originated proteins being in a ratio of less than 10ng of the proteins per 10,000U of the soluble thrombomodulin, wherein the soluble thrombomodulin is produced by a transformant cell obtained by transfecting a host cell with a DNA containing a nucleotide sequence encoding the soluble thrombomodulin.