Left and right sides of the mesh sandwiching the placing region therebetween are set as left and right to-be-fixed portions to be fixed to sample holder separation portions respectively to be moved in a stretch direction. A slit for dividing use is formed from a portion of the periphery of the mesh disposed between the left and right to-be-fixed portions thereof toward the rubber slice-placing position of the mesh in a direction orthogonal to the stretch direction of the rubber slice or a direction inclined thereto. When the to-be-fixed portions are moved in a separation direction by moving the sample holder separation portions, the mesh is divided into left and right parts by the slit for dividing use so that the rubber slice fixed to the left and right sides of the mesh is stretched.