A method and an apparatus for balancing a load by taking into consideration a load of each mobile communication network (Public Land Mobile Network (PLMNs)) of a network sharing architecture are provided. A base station apparatus of a communication system having a network sharing architecture capable of serving at least one mobile terminal via at least two PLMNs is provided. The apparatus includes a load calculator and a load balancing controller. The load calculator is configured to calculate respective loads of the PLMNs. When a first network of the PLMNs is overloaded, the load balancing controller is configured to balance an overload to the first network included in cells neighboring a serving cell.