A device for the measurement of the flow rate of a gas in a main gas piping, comprising: - first branch means, which in turn comprise a first branch pipe, developing inside a main piping according to a second direction at least in part transversal to a first direction of said main piping; said first branch pipe being configured for the diversion of a portion of a gas flow in transit in the main piping towards measuring means, - measuring means, comprising an inlet mouth, connected to the first branch pipe, and an outlet mouth, said measuring means being configured for the direct measurement of the flow rate of the portion of gas flow, diverted from the main piping, which crosses the same measuring means, - and second branch means, which in turn comprise a second branch pipe connected to the outlet mouth and configured for the re-introduction into the main piping of the diverted portion of gas flow crossing the measuring means, - said first branch pipe and said second branch pipe being concentric, said first branch pipe, comprising an external sampling tube, developing according to a second direction orthogonal to the first direction, said sampling tube crossing the wall of said main piping at a through hole defined on the same wall, - said sampling tube having at a first end a connecting section for the connection with said measuring means, - said sampling tube having a second end, opposite to the first end, closed, and being positioned in such a way that said second end is close to or in contact with the internal surface of said wall, - said sampling tube having three lateral sampling holes (121, 122, 123), said lateral sampling holes (121, 122, 123) being open, and turned, in the direction opposite to the direction of a gas flow in the main piping, - said second branch pipe comprising a return tube, developing according to a third direction orthogonal to said first direction and coinciding with said second direction, characterized in that - a lateral sampling hole of the sampling tube is placed at the axis of the main piping, - said second branch pipe has a lateral return hole turned in the same direction of said gas flow, said lateral return hole developing along the same axis as the main piping, - said lateral return hole comprising a tubular section positioned between said return tube and said sampling tube, so as to create a passage between the second branch pipe and the inside of said main piping. - first branch means, which in turn comprise a first branch pipe, developing inside a main piping according to a second direction at least in part transversal to a first direction of said main piping; said first branch pipe being configured for the diversion of a portion of a gas flow in transit in the main piping towards measuring means, - measuring means, comprising an inlet mouth, connected to the first branch pipe, and an outlet mouth, said measuring means being configured for the direct measurement of the flow rate of the portion of gas flow, diverted from the main piping, which crosses the same measuring means, - and second branch means, which in turn comprise a second branch pipe connected to the outlet mouth and configured for the re-introduction into the main piping of the diverted portion of gas flow crossing the measuring means, - said first branch pipe and said second branch pipe being concentric, said first branch pipe, comprising an external sampling tube, developing according to a second direction orthogonal to the first direction, said sampling tube crossing the wall of said main piping at a through hole defined on the same wall, - said sampling tube having at a first end a connecting section for the connection with said measuring means, - said sampling tube having a second end, opposite to the first end, closed, and being positioned in such a way that said second end is close to or in contact with the internal surface of said wall, - said sampling tube having three lateral sampling holes (121, 122, 123), said lateral sampling holes (121, 122, 123) being open, and turned, in the direction opposite to the direction of a gas flow in the main piping, - said second branch pipe comprising a return tube, developing according to a third direction orthogonal to said first direction and coinciding with said second direction, - a lateral sampling hole of the sampling tube is placed at the axis of the main piping, - said second branch pipe has a lateral return hole turned in the same direction of said gas flow, said lateral return hole developing along the same axis as the main piping, - said lateral return hole comprising a tubular section positioned between said return tube and said sampling tube, so as to create a passage between the second branch pipe and the inside of said main piping.