A grinding device for gate valves which can be attached in a simple manner to the interior of a valve seat. The valve can by a simple rotation back and forth of the grinding device, e.g. by means of cord drive, be ground very accurately by hand. By adjusting press members provided on the device, the grinding ring can also be made to press with varying degrees of force against the respective parts of the valve, such that the grinding effect can be regulated. Because the grinding ring is designed to use removable abrasive paper, it is also very simple first to carry out a rough grinding and thereafter to perform fine grinding. Adaptation of the device to valves of different sizes is simplified.