A method of operating a wind turbine generator which includes a wind turbine rotor including a hub having a blade mounted to the hub, a generator configured to be driven by rotation energy of the wind turbine rotor, a drivetrain for transmitting the rotation energy of the wind turbine rotor to the generator, the drivetrain including at least one oil using unit, a storage tank for storing oil supplied to the oil using unit, and a circulation line through which oil circulated between the storage tank and the oil using unit flows, the method comprising: an oil-leakage determination step of determining whether oil leakage from the oil using unit has occurred, using a first oil-leakage determination condition including both of following conditions a) and b); a wind-turbine stop step of stopping the wind turbine generator if it is determined that the oil leakage has occurred in the oil-leakage determination step; and an auxiliary-machine stop step of stopping an auxiliary machine of the wind turbine generator if it is determined that the oil leakage has occurred in the oil-leakage determination step, wherein, in the oil-leakage determination step, determination of condition satisfaction is carried out for each of conditions (a) and (b), and then it is determined that the oil leakage from the oil using unit has occurred if any one of the following condition a) or b) is not satisfied: condition a): a fluid level of the oil in the storage tank is greater than a threshold value; condition b): a pressure in the circulation line is greater than a threshold value, or a pressure in the oil using unit is greater than a threshold value, the method being characterized in that, if it is determined that the oil leakage has occurred according to the first oil-leakage determination condition in the oil-leakage determination step, the auxiliary-machine stop step is executed after a rotation speed of the main shaft of the wind turbine generator or of a generator shaft has decreased to a threshold value or less by executing the wind-turbine stop step. an oil-leakage determination step of determining whether oil leakage from the oil using unit has occurred, using a first oil-leakage determination condition including both of following conditions a) and b); a wind-turbine stop step of stopping the wind turbine generator if it is determined that the oil leakage has occurred in the oil-leakage determination step; and an auxiliary-machine stop step of stopping an auxiliary machine of the wind turbine generator if it is determined that the oil leakage has occurred in the oil-leakage determination step, wherein, in the oil-leakage determination step, determination of condition satisfaction is carried out for each of conditions (a) and (b), and then it is determined that the oil leakage from the oil using unit has occurred if any one of the following condition a) or b) is not satisfied: condition a): a fluid level of the oil in the storage tank is greater than a threshold value; condition b): a pressure in the circulation line is greater than a threshold value, or a pressure in the oil using unit is greater than a threshold value, the method being characterized in that, if it is determined that the oil leakage has occurred according to the first oil-leakage determination condition in the oil-leakage determination step, the auxiliary-machine stop step is executed after a rotation speed of the main shaft of the wind turbine generator or of a generator shaft has decreased to a threshold value or less by executing the wind-turbine stop step.