PURPOSE: To obtain strong joining force with not requiring a special press die and not increasing a number of process of press working.

CONSTITUTION: It is a Joining device of metal plates 500A, 500B to join by assembling bent parts 510A, 510B with bending the edge parts of two metal plates 500A, 500B and by pressing these bent parts 510A, 510B. The device is provided with a die 100, in which metal plates 500A, 500B assembled with bent parts 510A, 510B are mounted, a work holding plate 200 to hold both metal plates 500A, 500B mounted on the die 100 and a punch 300 to piercing a piercing hole 210 arranged in a work holding plate 200. A recessed part 110, to which the bent parts 510A, 510B assembled are press fitted, is formed in the die 100, a projecting tip part 310 of the punch 300 makes a part of bent parts 510A, 510B assembled press fit in the recessed part 110.