An imaging device includes: an area light source including an emission surface from which a sub-terahertz wave is emitted to a measurement target; and a detector including an image sensor that receives a reflected wave generated by the measurement target reflecting the sub-terahertz wave emitted from the emission surface. The area light source includes: at least one point light source that emits a sub-terahertz wave; and a reflector that reflects the sub-terahertz wave emitted from the at least one point light source, to generate a sub-terahertz wave to be emitted from the emission surface. The reflector has a reflection surface that is a bumpy surface which includes two or more frequency components in a spatial frequency range and whose roughness curve element mean length RSm is at least 0.3 mm.