Abstract: An antenna device includes: a main substrate provided with a ground electrode extending in a Y-axis direction; and a sub-substrate mounted on the main substrate. The sub-substrate includes: an upper surface on which a first antenna element and a second antenna element are arranged side by side in the Y-axis direction; and a lower surface on which a mounting terminal portion is disposed. A recessed portion recessed toward the upper surface is formed in a central region (a region between the first antenna element and the second antenna element of the lower surface of the sub-substrate.

Abstract: A multilayer substrate in which dielectric layers are laminated, the multilayer substrate including: a first electrode formed on the dielectric layers; and a first ground electrode disposed so as to be opposite to the first electrode in a multilayer direction. In the multilayer substrate, a plurality of dielectric layers include a first layer and a second layer that are disposed between a layer in which the first electrode is formed and a layer in which the first ground electrode is formed. In the first layer, a filler having permittivity lower than permittivity of a base material forming the dielectric layer is not disposed. In the second layer, the filler is disposed in at least a part of a region where the first electrode and the first ground electrode overlap each other in planar view of the multilayer substrate from the multilayer direction.