In a coil antenna device, a multilayer structure includes non-magnetic sheets and magnetic sheets stacked on each other. A coil conductor is provided in the multilayer structure such that a portion of the magnetic material defines a magnetic core and such that a coil axis extends along a principle surface of the multilayer structure. The coil conductor includes a plurality of line conductors each of which extends on one principle surface side of the magnetic material, a plurality of line conductors each of which extends on the other principle surface side of the magnetic material, and a plurality of via-hole conductors extending in a thickness direction of the multilayer structure so as to be surrounded with the magnetic material. The plurality of via-hole conductors defines a coiled structure together with the line conductors.