Abstract: A helmet-attachable communication device includes a communication unit wirelessly connected to a mobile communication device and having a case externally provided with a plurality of keys and an expansion slot; a fixing unit externally arranged on the case for attaching the communication device to an outer side of a helmet; a microphone/speaker unit externally connected to internal circuits of the communication unit and mounted in the helmet; and an expansion unit inserted in the expansion slot. A motorcycle rider with the communication device attached to the helmet can use the mobile communication device during riding safely without the need of holding the same with one hand. With the expansion slot, other expanded functions can be added to the communication device according to the user's need, giving the communication device enhanced functionality and practicality.