To improve the performance reliability of a coil device.

A coil pattern section is formed on a substrate 2 for mounting electronic component and, at the same time, core leg insertion through-holes 10 are formed in the formed area of the coil pattern section. By utilizing the insertion holes 10, a pair of core members are fitted to the substrate 2 so that the coil pattern section may be held between the core members from both the front and rear sides of the substrate 2. Then the core members are combined with each other by using core combining members. The opened marginal end faces of the through-holes 10 are formed in wave shapes. The core members are positioned and fixed to the substrate 2, by bringing the core legs of the core members into contact with the crests of the waves. The fraying out of the fibers of a fiber layer embedded in the substrate 2 from the opening end faces of the through-holes 10 can be prevented, by making the waves of the opened peripheral end faces of the holes 10 intersect each other in the longitudinal and transverse directions of the fibers.