A multi-stage gear box (122) of a wind power plant, wherein the gear box (122) includes a gear box housing (125). The gear box (122) includes at least one access opening for the insertion of a work instrument into the gear box housing (125) or respectively into the interior of the gear box housing (125) is provided on the outside of the gear box housing (125) and in that at least one guiding device (201, 202, 203, 204, 205) is provided for the work instrument inside the gear box housing (125) so that the work instrument is moveable and/or positionable within the guiding device (201, 202, 203, 204, 205). Parts of the gear box (122) in the gear box housing (125) are checked and/or worked by a work instrument inserted or arranged or arrangeable in the guiding device (201, 202, 203, 204, 205).