The present invention aims at providing an instrument for capturing free thrombi that inhibits blood coagulation reaction at the stage of primary hemostasis, in which platelets are involved, and at the stage of coagulation thrombus formation, in which blood coagulation factors are involved, thereby securely capturing free thrombi and extending the available time of the instrument. The present invention provides an instrument for capturing free thrombi including a compound having an antithrombin activity immobilized on a surface(s) thereof and a method for capturing free thrombi, which method includes capturing free thrombi in blood in a living body using this instrument for capturing free thrombi.
