Provided is a method that enables an interior of a wind turbine blade to be safely inspected. A method of inspecting an interior of a wind turbine blade includes the steps of: placing, in the wind turbine blade, an inspection unit including a support frame, at least one wheel rotatably provided to the support frame, and inspection equipment attached to a front portion of the support frame in a traveling direction; and conveying the inspection unit from a blade root portion toward a blade tip portion of the wind turbine blade. The conveying step includes connecting at least one extension bar to a back end portion of the inspection unit, and sending the inspection unit by pushing the extension bar toward the blade tip portion.