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 the thus produced composite material yields a high-molecular-weight matrix component that can form a thrombus when contacting with blood. It is now 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 with 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.
