example 45 degree with an outer surface thereof which substantially corresponds with the conical surface 12 b of the clamp member 12 the conical end 13 b has a tip ring portion 13 a at an end tip thereof a diameter of the tip ring portion 13 a is substantially equal to an inner diameter of the multi channel tube 1 a machining of the end of the multi channel tube 1 a is processed with the aforementioned clamp member 12 and punching rod 13 as described below the end of the multi channel tube 1 a is supported in the support hole 12 a of the clamp member 12 as shown in fig5 next the punching rod 13 is inserted into the support hole 12 a in the course of insertion the tip ring portion 13 a abuts and cuts the end of the partition 3 at borders between the partition 3 and the outer tube 2 as shown in fig6 thereby the cuts 6 are formed further the conical end 13 b presses and deforms the outer tube 2 separated from the partition 3 outward then the conical surface 13 b restricts widening of the outer tube 2 and hence the widened portion of the outer tube 2 takes a form of the flare portion 7 next the branching tubes 10 are respectively inserted into the channel holes 4 and 5 then the ends of the branching tubes 10.