In the vehicular ad hoc networks (VANET), various authentication schemes have been proposed for secure communications. However, the previous schemes are inefficient because each vehicle needs to share and keep a large number of session keys for communicating with the other vehicles on the VANET. To overcome the above drawback, we propose a new authenticated communication scheme for the VANET. In the proposed scheme, each vehicle communicates with the other vehicles through the roadside unit (RSU). Based upon this environment, each vehicle only has to share a session key with the RSU to communicate with different vehicles. Thus, the proposed communication model can be simplified on the VANET.