Abstract: A carbon dioxide absorber (1) and a closed-circuit breathing apparatus (2) with the carbon dioxide absorber are based on the carbon dioxide absorber having an inlet (3) and an outlet (4) gas-tight connectable by a flow duct (5), in which a material (6) is arranged, which absorbs some carbon dioxide contained in the breathing gas stream sent through the material. The flow duct (5) is enclosed in some areas by a housing (7), in which a window element (8) is arranged. A display element (9) arranged movably in the flow duct (5) is visible through the window element from outside of the housing and/or through which window element the radiation reflected by the display element (9) exits to the outside. A distance between the window element (8) and the display element (9) varies as a function of the quantity of carbon dioxide-absorbing material arranged in the flow duct (5).