A plurality of clad rods, and a clad tube, an arrangement process for arranging the plurality of core rods and the plurality of clad rods in a tube of the clad tube, in a state in which distances between center axes of the adjacent core rods become equal to each other and a state in which parts of outer circumferential surfaces in the adjacent rods contact, and an integration process for integrating the clad tube and the plurality of core rods and the plurality of clad rods arranged in the tube, wherein a ratio of a total cross-sectional area of a direction orthogonal to a length direction in the plurality of core rods and the plurality of clad rods with respect to an internal cross-sectional area of the tube of a direction orthogonal to a length direction in the clad tube is 0.84 or more.