Abstract: The viewing window at the front of a breathing mask is held in a groove in a flexible frame that is clamped in a ring formed from two channel-shape half rings engaging each other at their ends and having on the outside of their end portions pairs of laterally projecting vertical flanges so that those end portions are H-shaped in cross section. Each pair of the engaging ends of the half rings is provided with a guide member projecting vertically from one of the half rings into the space between the flanges of the other half ring, and a bolt is rotatably disposed in a cross member between the flanges of the other half ring. The bolt extends through the adjacent guide member and has a nut on its end so that the ends of each half ring can be drawn into contact with the ends of the other half ring to hold the half rings together. The inner surfaces of the side walls of the half rings beside the bolts may diverge toward the ends of the rings.