The present invention with the device (B) for monitoring the access to the patient for the device to a patient via a hose line fluid is supplied and / or the fluid forced out from the patient, especially for extracorporeal blood treatment apparatus (A) It relates to a moisture detection device 40 for use. Moisture detecting device according to the invention is configured as a two-dimensional extended fabric is placed on the skin surface of the patient, the two-dimensional extension fabric comprises an electrically conductive structure as a water sensor. The extension is a two-dimensional fabric extending fabric-type two-dimensional fabric, which is non-being configured by both conductive as well as conductive warp and weft conductive weft yarns (50, 60) and the non-conductive warp yarns. Conductive and non-conductive warp and weft yarns are arranged in such a manner that generates the electrically conductive structure. Through the spatial separation of the warp and weft yarns it is made of a limited electrical conductivity structure in the straight molding fabric. Decisive advantages of using conductive and conductive warp and weft both lies in the fact that the electrically conductive structure that includes each section leads in the other direction can be configured. By such a structure it may be made in particular the water sensor with a high sensitivity. There is an electrically conductive structure is preferably closed by a terminating resistor (R), which together that the connection parts for the apparatus is not a component of the moisture detection device 40 is preferred, whereby the manufacture is simplified accordingly.