A method for drilling holes into a mantle of a cylinder of a paper machine while the cylinder is in its site of operation in the paper machine. The holes are drilled into the cylinder mantle by a drilling device having drill bits and which is transferred after each drilling cycle to the next drilling position. When drilling the holes, the device is supported on cylinders placed adjacent to and above the cylinder to be machined and/or on the cylinder to be machined by fastening supports of the device. Before the drilling is started, the device is locked in its position by locking fastening supports in a desired position and, the holes are drilled into the cylinder placed underneath from the top toward the bottom. The device includes drill bits, a frame construction, a drive gear for transferring and rotating the drill bits during the drilling, fastening supports for supporting the device in its place in the drilling position against the cylinders placed above and adjacent to the cylinder to be machined and/or against the cylinder to be machined, and an actuator for locking the device in its place in the drilling position.