Recently, WiBro has been developed to efficiently support broadcasting via wireless network. MBS (Multicast Broadcast Service) based on WiBro has numerous applications including DMB. Since the current MBS implementation uses only common channel which is managed by scheduling, it cannot effectively support many users requesting the service at the same time. This paper proposes a new counting scheme with which the channel mode can be properly switched between common channel and dedicated channel by considering the number of users and their mobility. It thus allows efficient use of limited radio resource in WiBro supporting the MBS. The simulation results with NS-2 show that it is significantly more efficient than the existing scheme when many users request the same broadcasting service channel.