The invention relates to a device for joining thin, flat elements (12a, 12b) by means of an adhesive, comprising a transport unit (9) which is used to transport the flat elements (12a, 12b) through the device, and a device (2) for producing microwaves in order to harden an adhesive which is applied to the edges of the flat elements (12a, 12b). The invention is characterised in that the microwaves are guided into a wave guide (3) which comprises at least one slot (4). The butt joint (13) of the flat elements (12a, 12b) comprising the adhesive can be guided past the slit (4) in order to harden the adhesive by means of the microwaves emerging from the slit (4). The invention also relates to a method for joining thin, flat elements.