A detecting device for detecting a usage state of a socket includes a carrier, a movable assembly and a conductivity detecting module. The carrier has a first plug hole and a second plug hole. The movable assembly is movably disposed in the carrier. The conductivity detecting module is disposed on the carrier. The movable assembly is moved for exposing the first plug hole and the second plug hole by an external pushing force, and a detecting signal of the socket in-use is generated by the conductivity detecting module according to movement of the movable assembly.