Abstract: This invention relates to breathing apparatus specially suited to submerged use, of the kind comprising a two-stage pressure reducer whose first stage is connected to the outlet valve of the diving cylinder and is responsible for the reduction from high pressure to medium pressure, while the second stage is associated directly with the mouthpiece and supplied it with air for respiration in response to each inhalatory suction from the user which exceeds a minimum threshold value termed the lifting suction.According to the invention, the first stage includes structure for adjusting the medium pressure, which has a member for direct manual operation which is mounted to be easily accessible and which can be operated by the user even when diving, so that the medium pressure can be adjusted whenever the lifting suction becomes too great, or whenever the effect of changes in the operation of the first stage is to alter it, or before diving in shallows.