Device for thermal joining of respective two material sheets (MB1, MB2) over a strip-shaped connecting piece lying areally on a bottom side, wherein the material sheets (MB1, MB2) are arranged with a respective side surface facing one another abutting against each other at least partially with a respective bottom side (US1, US2) lying areally on a top side of the connecting piece, wherein the device comprises a base body with a front edge, a rear edge opposite the front edge and two opposite longitudinal edges, wherein a top surface side and a bottom surface side opposite the top surface side of the base body respectively extend between the front edge and the rear edge as well as between the two longitudinal edges, wherein the top surface side, the bottom surface side, the front edge, the rear edge and the two longitudinal edges entirely border a volume of the base body, wherein the device with the base body is configured to be guided between the top side of the connecting piece and the bottom sides (US1, US2) of the material sheets (MB1, MB2) with the top surface side facing towards the bottom sides (US1, US2) of the material sheets (MB1, MB2) and the bottom surface side facing towards the top side of the connecting piece, wherein the base body is configured to guide the material sheets (MB1, MB2) at least in areas of their respective bottom sides (US1, US2) adjacent to their side surfaces from the front edge to the rear edge over the top surface side such that the material sheets (MB1, MB2) are guided starting from the front edge at least over a quarter, in particular at least over a half, in particular at least over two-thirds of a maximum extension of the base body between the front edge and the rear edge with their respective side surfaces facing one another separated from one another, and wherein the device is configured to simultaneously heat at least partially the bottom side (US1, US2) of the material sheets (MB1, MB2), the top side of the connecting piece and the respective side surface of the material sheets (MB1, MB2) facing one another so as to produce respectively between each of the two material sheets (MB1, MB2) and the connecting piece as well as between the material sheets (MB1, MB2) parallel connections, in particular parallel weld seams; characterized in that the base body is configured as a hollow body and comprises at least one opening for outlet of hot air from the hollow body at the rear edge of the base body into a space between the rear edge of the base body, the top side of the connecting piece and the bottom sides (US1, US2) of the material sheets (MB1, MB2). wherein the device comprises a base body with a front edge, a rear edge opposite the front edge and two opposite longitudinal edges, wherein a top surface side and a bottom surface side opposite the top surface side of the base body respectively extend between the front edge and the rear edge as well as between the two longitudinal edges, wherein the top surface side, the bottom surface side, the front edge, the rear edge and the two longitudinal edges entirely border a volume of the base body, wherein the device with the base body is configured to be guided between the top side of the connecting piece and the bottom sides (US1, US2) of the material sheets (MB1, MB2) with the top surface side facing towards the bottom sides (US1, US2) of the material sheets (MB1, MB2) and the bottom surface side facing towards the top side of the connecting piece, wherein the base body is configured to guide the material sheets (MB1, MB2) at least in areas of their respective bottom sides (US1, US2) adjacent to their side surfaces from the front edge to the rear edge over the top surface side such that the material sheets (MB1, MB2) are guided starting from the front edge at least over a quarter, in particular at least over a half, in particular at least over two-thirds of a maximum extension of the base body between the front edge and the rear edge with their respective side surfaces facing one another separated from one another, and wherein the device is configured to simultaneously heat at least partially the bottom side (US1, US2) of the material sheets (MB1, MB2), the top side of the connecting piece and the respective side surface of the material sheets (MB1, MB2) facing one another so as to produce respectively between each of the two material sheets (MB1, MB2) and the connecting piece as well as between the material sheets (MB1, MB2) parallel connections, in particular parallel weld seams; characterized in that the base body is configured as a hollow body and comprises at least one opening for outlet of hot air from the hollow body at the rear edge of the base body into a space between the rear edge of the base body, the top side of the connecting piece and the bottom sides (US1, US2) of the material sheets (MB1, MB2).