A circuit breaker comprising: an airtight container filled with a dielectric gas; a stationary frame arranged inside the container defining a volume with a main axis; at least a pair of arc contacts, the arc contacts being separable from each other; an actuation rod that drives one of the arc contacts in a separation direction along the main axis inside the volume defined by the frame, upon a separation operation between the arc contacts; a compression chamber in which a portion of the dielectric gas is compressed during the separation operation to blast an electric arc formed between the arc contacts during the separation operation; a cylinder and a piston connected with the rod, which are movable within the stationary frame along main axis, for compressing said portion of the dielectric gas in the compression chamber upon the separation operation; a link mechanism connecting the piston to the rod, wherein the stationary frame comprises a cylindrical portion with which each of the piston and of the cylinder are radially in gastight contact and which bounds the compression chamber together with the piston and the cylinder; characterized in that the cylindrical portion of the stationary frame comprises an orifice which communicates with the internal volume of the compression chamber through the piston for allowing dielectric gas to fill the compression chamber during a closing operation of the circuit breaker, and that the piston comprises two radial collars (40, 42) which are in a gastight cooperation with the cylindrical portion of the stationary frame, which are axially distant one with respect to the other, wherein the radial collars (40, 42) axially bound an annular chamber which said annular chamber is in communication with said orifice of the cylindrical portion of the stationary frame. an airtight container filled with a dielectric gas; a stationary frame arranged inside the container defining a volume with a main axis; at least a pair of arc contacts, the arc contacts being separable from each other; an actuation rod that drives one of the arc contacts in a separation direction along the main axis inside the volume defined by the frame, upon a separation operation between the arc contacts; a compression chamber in which a portion of the dielectric gas is compressed during the separation operation to blast an electric arc formed between the arc contacts during the separation operation; a cylinder and a piston connected with the rod, which are movable within the stationary frame along main axis, for compressing said portion of the dielectric gas in the compression chamber upon the separation operation; a link mechanism connecting the piston to the rod, wherein the stationary frame comprises a cylindrical portion with which each of the piston and of the cylinder are radially in gastight contact and which bounds the compression chamber together with the piston and the cylinder; characterized in that the cylindrical portion of the stationary frame comprises an orifice which communicates with the internal volume of the compression chamber through the piston for allowing dielectric gas to fill the compression chamber during a closing operation of the circuit breaker, and that the piston comprises two radial collars (40, 42) which are in a gastight cooperation with the cylindrical portion of the stationary frame, which are axially distant one with respect to the other, wherein the radial collars (40, 42) axially bound an annular chamber which said annular chamber is in communication with said orifice of the cylindrical portion of the stationary frame.