In Proxy Mobile IPv6 (PMIPv6), a new entity called Mobile Access Gateway (MAG) performs the mobility-related signaling with Local Mobility Anchor (LMA) on behalf of MNs (Mobile Nodes). However, a number of MNs must be associated with a MAG and hence, the MAG can be easily overloaded. Thus, in this paper, we propose a load balancing mechanism for the PMIPv6 network. We also discuss about using IEEE 802.21 Media Independent Handover (MIH) protocol in the load balancing to learn the load status at the candidate Point of Attachments (PoAs), in addition to the load status at the candidate MAGs. It is shown via numerical and simulation results that the proposed load balancing mechanism can realize lower queuing delay at the MAG and higher data transmission rate at the PoA in the PMIPv6 network.