To provide a header mounting device capable of securely mounting a header for heat exchanger on a tube.

The header mounting device comprises a slide body 109 which can freely slide along a rail 115, a first positioning member 110 for heat exchanger header and a second positioning member for heat exchanger header 111 to be fixed on the slide body 109, a first clamping member 112 for heat exchanger header for clamping a first header 7 for heat exchanger and pressing it against the first positioning member 110 for heat exchanger header, and a second clamping member 113 for heat exchanger header for clamping a second header 6 for heat exchanger and pressing it against the second positioning member 111 for heat exchanger header, and a longitudinal driving means for moving the slide body 109 forward and backward so that it is close to and separate from the tube.