The present invention discloses a method for determining the efficacy of GPC3-targeting drug therapy for cancer in a patient before the start of GPC3-targeting drug therapy or a patient or determining the continuation of GPC3-targeting drug therapy for a patient, including monitoring a concentration of free GPC3 in a biological sample isolated from the patient before the start of GPC3-targeting drug therapy and/or the patient treated with the GPC3-targeting drug therapy, wherein when the concentration of free GPC3 is a predetermined value, the efficacy of the GPC3-targeting drug therapy is determined or the continuation of the GPC3-targeting drug therapy is determined. The present invention also discloses a GPC3-targeting drug or a preparation which is to be further administered to a patient for which the efficacy of the GPC3-targeting drug therapy has been determined or the continuation of the GPC3-targeting drug therapy has been determined.
