A tube for a heat exchanger includes a first and a second plate portions disposed opposite each other. At least the first plate portion includes a base plate portion bonded to the second plate portion by brazing and multiple protruding portions, which protrude from the base plate portion toward an opposite side from the second plate portion, having a first and a second protruding portions. A protruding dimension of the first protruding portion is larger than that of the second protruding portion. The base plate portion and the protruding portions extend straightly in a tube longitudinal direction. The second plate portion and the protruding portions have a clearance therebetween defining an internal fluid passage. The second protruding portion and the base plate portion are configured to define an external fluid passage extending in a tube width direction.