A method of diagnosing a wind turbine power generating apparatus, comprising: an operation step of operating a pitch actuator corresponding to one of a plurality of wind turbine blades of the wind turbine power generating apparatus, without operating a pitch actuator corresponding to each of the wind turbine blades other than the one of the plurality of wind turbine blades; and a measurement step of measuring a response value indicating a response to operation of the pitch actuator corresponding to the one of the plurality of wind turbine blades, wherein the operation step and the measurement step are performed repeatedly for each of the plurality of wind turbine blades, and the response value is obtained as data for health check of a blade pitch mechanism of the wind turbine power generating apparatus, characterized in that the method comprises: a step of, after operating the pitch actuator corresponding to one of a plurality of pitch accumulators corresponding to the plurality of wind turbine blades to a feather side with pressurized oil supplied from the one of the pitch accumulators, measuring a pressure of oil in a pitch oil line including the one of the pitch accumulators disposed therein and a gas pressure of the one of the pitch accumulators, during a period in which the pitch actuator is operated toward a fine side or the feather side only with the pressurized oil supplied from the one of the pitch accumulators; and a step of obtaining the gas pressure at a point of time when a difference between the gas pressure and a pressure of the oil reaches a set value or a later appropriate point of time when the difference is not smaller than the set value, as the data for health check of the blade pitch mechanism. an operation step of operating a pitch actuator corresponding to one of a plurality of wind turbine blades of the wind turbine power generating apparatus, without operating a pitch actuator corresponding to each of the wind turbine blades other than the one of the plurality of wind turbine blades; and a measurement step of measuring a response value indicating a response to operation of the pitch actuator corresponding to the one of the plurality of wind turbine blades, wherein the operation step and the measurement step are performed repeatedly for each of the plurality of wind turbine blades, and the response value is obtained as data for health check of a blade pitch mechanism of the wind turbine power generating apparatus, characterized in that the method comprises: a step of, after operating the pitch actuator corresponding to one of a plurality of pitch accumulators corresponding to the plurality of wind turbine blades to a feather side with pressurized oil supplied from the one of the pitch accumulators, measuring a pressure of oil in a pitch oil line including the one of the pitch accumulators disposed therein and a gas pressure of the one of the pitch accumulators, during a period in which the pitch actuator is operated toward a fine side or the feather side only with the pressurized oil supplied from the one of the pitch accumulators; and a step of obtaining the gas pressure at a point of time when a difference between the gas pressure and a pressure of the oil reaches a set value or a later appropriate point of time when the difference is not smaller than the set value, as the data for health check of the blade pitch mechanism. a step of, after operating the pitch actuator corresponding to one of a plurality of pitch accumulators corresponding to the plurality of wind turbine blades to a feather side with pressurized oil supplied from the one of the pitch accumulators, measuring a pressure of oil in a pitch oil line including the one of the pitch accumulators disposed therein and a gas pressure of the one of the pitch accumulators, during a period in which the pitch actuator is operated toward a fine side or the feather side only with the pressurized oil supplied from the one of the pitch accumulators; and a step of obtaining the gas pressure at a point of time when a difference between the gas pressure and a pressure of the oil reaches a set value or a later appropriate point of time when the difference is not smaller than the set value, as the data for health check of the blade pitch mechanism.