The gasometric bell with telescopic guides is used on sludge treatment plants when it is necessary to store and at the same time supply at constant pressure the gas produced by the anaerobic decomposition of the sludge. It consists of a cylindrical bell which vertically slides on right guides and a cylindrical concrete tank which is the stationary part of the gasometer wherein the bell slides. The volumetric variations of the gasometer allow variable flows of the inlet gases coming from the anaerobic digesters and also variable flows, but at constant pressure, of the outlet gas to the distribution network. The bell top is provided with a couple of manholes, a vacuum-breaking valve and a safety valve with flame trap. Moreover the concrete tank is equipped with a ladder to approach the roof, while the roof and the bell have a safety handrail all along the higher perimeter. The gasometer is provided with a level glass with local alarm and level switch.