The present invention provides a new method for airtightly joining a hollow tube made of a reinforced platinum and a flange made of platinum or a platinum alloy, which prevents decreasing of mechanical strength of the hollow tube made of a reinforced platinum due to its joining with a flange. A method for airtightly joining a hollow tube made of a reinforced platinum with a flange made of platinum or a platinum alloy, which is a method for airtightly joining the outer periphery of a hollow tube made of a reinforced platinum produced by dispersing metal oxides in platinum or a platinum alloy, with a flange made of platinum or a platinum alloy, the method comprising a step of shrink-fitting a ring made of platinum or a platinum alloy to the outer periphery of the hollow tube, a step of heating the hollow tube to which the ring is shrink-fit at from 1,150 to 1,450° C. for from 1 to 10 hours, and a step of welding the flange made of platinum or a platinum alloy to the outer periphery of the ring.