To solve a problem in a conventional method for forming a passive state film on an inside surface of a metal pipe in which joints at a front end and a rear end of the metal pipe are connected by caulking one by one, resulting in rendering working efficiency poor.

The method for gas treatment of the inside surface of the metal pipe made to be moved inside a furnace comprises longitudinally moving the metal pipe in a continuous length inside the furnace, and supplying gas into the inside of the metal pipe at the same time to treat the inside surface of the metal pipe. One set of a preceding stage where a plurality of metal pipes are arranged in parallel is connected by one set of a following stage and a joint fixture, and these sets are successively conveyed in. The gas to be supplied from the joint fixture outside a conveying-in side of the furnace is made to flow in a moving direction of the metal pipes through a plurality of the insides of the pipes and the insides of the joint fixtures. The gas is exhausted from the joint fixture outside a conveying-out side of the furnace.