Disclosed are a mobile device, an HMD and a system. A mobile device which is capable of data communication with an HMD, according to one embodiment of the present invention, comprises: a touch sensing module which is designed to recognize a touch from a user; a communication module which performs data communication with a head-mounted display (HMD); a video processor which if relative location information between the mobile device and the HMD is received from the HMD, generates extended video data on the basis of the location information; and a controller which after the generated extended video data is transmitted to the HMD via the communication module, controls the extended video data output by the HMD.