A terahertz wave detecting device includes: a substrate; a first metal layer that is disposed above the substrate; a pyroelectric layer that is disposed on the first metal layer; and a second metal layer that is disposed on the pyroelectric layer, wherein the second metal layer has a periodic structure in which a unit structure is disposed in a predetermined period, and the pyroelectric layer absorbs terahertz waves being incident on the pyroelectric layer and converts the terahertz waves into heat and converts the converted heat into an electrical signal.