Pressure reducing second stage for underwater use comprising: - a case provided with a breathing gas inlet and with an inhalation/exhalation port through which the gas supplied into the case is inhaled by the user and through which the user supplies the exhaled gas into the case, said case being provided with an exhaled gas exhaust outlet with a non-return valve, - the gas inhalation/exhalation port being connected to an inhalation/exhalation mouthpiece tube through which the breathing gas is inhaled by the user and through which the gas is exhaled into the case, - the breathing gas inlet in the case being connected to a breathing gas pressure reducing device provided with a hollow cylindrical element or pipe, which hollow cylindrical element has a breathing gas inlet port communicating, through a valve, with a gas supply conduit, which valve is interposed between said supply conduit and said inhalation/exhalation mouthpiece tube for said breathing gas, - said case being provided with pressure sensitive means for controlling the opening/closure of said wherein the breathing gas inlet in the case is provided in the lower portion of said case, considering the second stage used by a user in the upright position and said hollow cylindrical element of the pressure reducing device being connected to said inlet on the case such that its axis is vertical or substantially vertical, with reference to the upright position of the user, that is substantially parallel to the longitudinal axis, or cranio-caudal axis, that is the intersection of the sagittal plane with the coronal plane of the user body; and in that there is provided a tube, so called bypass tube, supplying said breathing gas from the pressure reducing device directly into the inhalation/exhalation mouthpiece tube, upon the opening of the pressure reducing valve, the bypass tube being connected, at one end thereof, to an port provided on the shell wall of the hollow cylindrical element of the pressure reducing device, and at the opposite end to an port provided on the shell wall of the inhalation/exhalation mouthpiece tube; characterized in that in the inhalation/exhalation mouthpiece tube there is provided a tongue placed substantially diametrically with orientation transverse to the axis of the bypass tube and with extension substantially parallel to the axis of said inhalation/exhalation mouthpiece tube, and said tongue axially extends from a position where it substantially overlaps the outlet port of the bypass tube in the inhalation/exhalation mouthpiece tube, with one end thereof facing towards the inside of the pressure reducing second stage, up to the opposite end region of said inhalation/exhalation mouthpiece tube, that is facing the user and intended to be coupled to a mouthpiece element. - a case provided with a breathing gas inlet and with an inhalation/exhalation port through which the gas supplied into the case is inhaled by the user and through which the user supplies the exhaled gas into the case, said case being provided with an exhaled gas exhaust outlet with a non-return valve, - the gas inhalation/exhalation port being connected to an inhalation/exhalation mouthpiece tube through which the breathing gas is inhaled by the user and through which the gas is exhaled into the case, - the breathing gas inlet in the case being connected to a breathing gas pressure reducing device provided with a hollow cylindrical element or pipe, which hollow cylindrical element has a breathing gas inlet port communicating, through a valve, with a gas supply conduit, which valve is interposed between said supply conduit and said inhalation/exhalation mouthpiece tube for said breathing gas, - said case being provided with pressure sensitive means for controlling the opening/closure of said wherein the breathing gas inlet in the case is provided in the lower portion of said case, considering the second stage used by a user in the upright position and said hollow cylindrical element of the pressure reducing device being connected to said inlet on the case such that its axis is vertical or substantially vertical, with reference to the upright position of the user, that is substantially parallel to the longitudinal axis, or cranio-caudal axis, that is the intersection of the sagittal plane with the coronal plane of the user body;