Recently, the total number of distributed generation such as photovoltaic generation system and wind turbine generation system connected to distribution network is drastically increased. Distributed generation utilizing renewable energy can reduce the distribution loss and emission of CO₂. However the distribution network with the distributed generators must be operated keeping reliability of power supply and power quality. In this paper, the authors propose a computation method to determine the maximum output of a distributed generator under the operational constrains ((1) voltage limit, (2) line current capacity, and (3) no reverse flow to bank) at arbitrary connection point and hourly period. In the proposed method, three-phase iterative load flow calculation is applied to evaluate the above operational constraints. Three-phase iterative load flow calculation has two simple procedures: (Procedure1) addition of load currents from terminal node of feeder to root one, and (Procedure2) subtraction of voltage drop from root node of feeder to terminal one. In order to check the validity of the proposed method, numerical simulations are accomplished for a distribution system model. Furthermore, characteristics of locational and hourly maximum output of distributed generator connected to distribution feeder are analyzed by several numerical examples.