A device of oscillating an electromagnetic wave having a frequency of 0.1 THz to 3 THz from pump and idler waves by parametric effect. The device includes a supporting body, an oscillating substrate of a non-linear optical crystal, an adhesive layer adhering the supporting body and the oscillating substrate, and a film for reflecting the electromagnetic wave formed on a surface of the supporting body on the side of the adhesive layer. The oscillating substrate has an upper face, a bottom face and an incident face on which the pump wave is made incident, with the adhesive layer having a refractive index with respect to the pump wave lower than that of the oscillating substrate.