An apparatus for detecting electromagnetic wave includes an electromagnetic wave sensor, a first electrode and a second electrode spaced from each other and electrically connected to the electromagnetic wave sensor, and a measuring device electrically connected to the first electrode and the second electrode. The electromagnetic wave sensor includes a carbon nanotube structure. The carbon nanotube structure includes a plurality of carbon nanotubes extending along a same direction from the first electrode to the second electrode. The measuring device is capable of measuring resistance of the carbon nanotube structure.