A low-temperature joining method effectively suppresses reductions in the mechanical properties of a junction of various types of high-tensile steel or aluminum, and of a heat-affected zone; and produces a joint structure. A method for joining two metal materials by forming a joint interface in which the two metal materials face each other at a joint portion and plunge a rotation tool caused to rotate at a prescribed speed into the joint, the method for low-temperature joining of metal materials characterized in that the peripheral velocity of the outermost periphery of the rotation tool is set to 51 mm/s or less, whereby the recrystallization temperature inherent to the metal materials is reduced by introducing a large strain to the joint, and recrystallized grains are generated at the joint interface by setting the joining temperature to less than the recrystallization temperature inherent to the metal materials.