In a multilayer substrate, eight front-side antenna portions and eight back-side antenna portions are disposed. Front-side radiation elements in the front-side antenna portions and back-side radiation elements in the back-side antenna portions are arranged in a staggered pattern when being vertically projected onto an back side of the multilayer substrate. The front-side radiation elements are disposed on a front side of the multilayer substrate, and a front-side ground layer is formed near the back side of the multilayer substrate. On the other hand, the back-side radiation elements are disposed on the back side of the multilayer substrate, and a back-side ground layer is formed near the front side of the multilayer substrate. The front-side radiation element and the back-side radiation element are disposed so as not to overlap each other when being vertically projected onto the back side of the multilayer substrate.