Abstract: In a pressurized gas circuit-breaker the mobile contact member is fixed to a connecting rod that passes coaxially through the fixed braking cylinder. The braking cylinder has transverse walls at both ends so as to delimit a cylinder. The connecting rod has a drive abutment that drives the braking piston in a final section of the opening travel, which increases the pressure in the first sub-space and reduces the pressure in the second sub-space. The braking piston is disposed on a bush around the connecting rod, the bush and the connecting rod delimiting an annular cylinder space between the drive member and a counter-abutment on the bush, a pressure being generated when the circuit-breaker opens. In this way a high braking force with no impact is very rapidly obtained.