A detection device that is capable of being attached to a through hole includes a main body portion, a detection portion, a cover portion, an insulation portion, and an oscillation portion. The main body portion has an insertion portion and a head portion. The detection portion is arranged in the insertion portion and detects the state of a fluid. The cover portion is arranged so as to form a gap with the head portion. The insulating portion has a lateral wall having a cylindrical shape. An oscillation portion is accommodated inside the insulation portion, and performs a wireless output of a detection result of the detection portion by using the head portion, the cover portion and the gap as a slot antenna.