In a linear conductor forming step, a wide portion having a relatively large line width and a narrow portion having a relatively small line width are formed in each of a plurality of linear conductors. In addition, in a multilayer board, in base material layers adjacent to each other in a stacking direction, the wide portion overlaps the narrow portion on the adjacent base material layer, and end portions of the wide portions at both sides of the narrow portion in a line width direction, in a planar view. The wide portions are disposed such that the end portions thereof overlap each other in the stacking direction and resistance of a fluid thermoplastic resin increases. The narrow portion is located between the wide portions in the stacking direction.