A device for measuring the internal temperature of a reforming tube including a first structure having an axial part of tubular shape positioned in the lengthwise direction of a reforming tube and a radial part projecting radially towards the central axis of the reforming tube, a second structure of oblong shape having at least one thermocouple made of welded Nicrosil/Nisil conductors arranged longitudinally against the axial part and radially against the radial part, and an outer sheath enveloping the first structure and the second structure.