A terahertz device of the present invention includes a terahertz element generating an electromagnetic wave, a dielectric including a dielectric material and surrounding the terahertz element, a gas space including a gas, and a reflecting film serving as a reflecting portion. The reflecting film includes a portion opposing the terahertz element through the dielectric and the gas space and reflecting the electromagnetic wave toward a direction, wherein the electromagnetic wave is generated from the terahertz element and transmitted through the dielectric and the gas space. In addition, the refractive index of the dielectric is lower than the refractive index of the terahertz element and is higher than the refractive index of the gas in the gas space.