This paper proposes how to determine optimal power flow (OPF) to realize emergency islanding operation of distribution networks by using distributed generators (DGs) and electric vehicles (EVs). Since the purpose of the emergency islanding operation is to keep the power supply capability as long as possible, the most important strategy is minimization of energy loss, which consists of charge-discharge loss of batteries and ohmic loss in distribution lines. This paper proposes formulation of OPF problem to minimize the energy loss and solution algorithm, and that Lagrange multipliers are useful for determination of load shift to reduce the energy loss further.