Previously reported composite materials comprising a hydrophilic high-molecular-weight matrix and a low-molecular-weight organic compound involve a problem in that: it is difficult to compound a high load of the low-molecular-weight organic compound in the high-molecular-weight matrix; and thus produced composite material yields a high-molecular-weight matrix component that can form a thrombus when contacting with blood. Now, it becomes possible to produce a composite material having an antithrombogenic activity and a high content of a low-molecular-weight organic compound by the present invention comprising the steps of: crosslinking a polymer with an organic acid derivative which is used as a crosslinker in an organic solvent while at the same time embedding the low-molecular-weight organic compound therein; and substituting the organic solvent by water after the production of the high-molecular-weight matrix to effect precipitation and compounding of the low-molecular-weight organic compound in the high-molecular-weight matrix.