To provide a method for repairing crack, defect, etc., developed in a cast product.

In such portion developing the crack or the defect, a truncated cone shaped opening part is arranged in the product. A tapered plug fitted to the opening part is prepared. Such tapered plug is the one which is fittable into the opening part so that this inclined side wall is uniformly supported from the one side on the trancated conical shaped side wall of the opening part. Such tapered plug is set in the opening part and also, binder is added between the surface of the tapered plug and the inner surface of the side wall of the opening part before or after inserting the tapered plug into the opening part. Thereafter, the binder works to join the surface of the tapered plug and the inner surface of the side wall of the opening part by heating the product. Further, the outer end of the tapered plug is adjusted so as to become almost the same height as the outer wall of the product by grinding this outer end thereof. A controlled tight-fit state can be obtained by pushing the plug member into the opening part according to the suitable way of the application.