A reflective detection device, includes: oscillation element oscillating a terahertz wave; exit part from which the terahertz wave exits; incident part to which the terahertz wave reflected from a detection target is incident; and detection element detecting the terahertz wave incident to the incident part, wherein the exit part and the incident part are disposed on one side in first direction and are spaced apart from each other in second direction with respect to the detection target, the terahertz wave exiting from the exit part travels to propagate from the exit part toward the incident part in the second direction along a direction toward the detection object in the first direction, and the terahertz wave incident to the incident part travels to propagate from the exit part toward the incident part in the second direction along a direction away from the detection target in the first direction.