This paper proposes a MAC protocol for efficient broadcasting in wireless ad hoc networks. Pure flooding leads to serious redundant broadcasts, packet contention and packet collisions, known as the broadcast storm problem. This paper focuses on how to reduce the redundant broadcasts. Several protocols to achieve efficient broadcasting have been proposed. However a trade-off exists between the reachability and the broadcasting ratio, which is the ratio of the number of the broadcasting nodes to the number of received nodes. This paper proposes a new MAC protocol for on-demand broadcasting, which is referred to as dialogue-based protocol in order to resolve the trade-off. The dialogue-based protocol employs additional massages which are called as request packets and reply signals. They are exchanged in an on-demand manner, in order that relay candidate nodes recognize its neighbor nodes status whether they have already received the broadcast packets. Finally, by computer simulations, this paper presents that the dialogue-based scheme combines high reachability with low broadcasting ratio by using the low additional massages.