A terahertz device includes a terahertz element, a sealing resin, a wiring layer and a frame-shaped member. The terahertz element that performs conversion between terahertz waves and electric energy. The terahertz element has an element front surface and an element back surface spaced apart from each other in a first direction. The sealing resin covers the terahertz element. The wiring layer is electrically connected to the terahertz element. A frame-shaped member is made of a conductive material and arranged around the terahertz element as viewed in the first direction. The frame-shaped member has a reflective surface capable of reflecting the terahertz waves.