To obtain the device and its method in which the pipe is stably supported by a pipe supporting device for a laser beam machine even if the size of the pipe is changed.

In a pipe supporting device 19 of a laser beam machine 1 in which a pipe rotating device 17 for rotating a pipe 15 is provided on a base 13, and in which a laser machining head 7 is provided in the upper part of the base, the pipe supporting device is equipped with a slide 21 freely movable in the direction parallel to the pipe on the base. The slide 21 is provided with a pair of carriages 25 allowing them to approach and recede mutually in the direction orthogonally crossing the pipe 15, with each of the pair of carriages 25 provided with a member 39 for freely rotatably supporting the pipe.