absorbing upper circuit 6 and the shock absorbing lower circuit 7 respectively a semiconductor switching element 1 and a free wheel diode 2 which are bonded to the upper circuit 3 by soldering or sintering and a resin case 8 by which these elements are sealed the lower circuit 4 of the insulating substrate is connected to a thinned base 11 with fins 12 the thinned base 11 being served as a first metal base whose thickness is thinned to for example several mm to be low stiffened the lower circuit 4 being connected by a bonding layer 10 of solder or sintering material and the thinned base 11 is joined to a thickened and high stiffened second metal base 13 formed with water passages according to the semiconductor device of the thus configured embodiment 1 the shock absorbing lower circuit 7 serving as a stress relief layer is pure aluminum and thus bearing force is low and bearing force is equal to or lower even compared to the bearing force of the bonding layer 10 of solder or sintering material and therefore heat distortion generated in the bonding layer 10 can be reduced furthermore the insulating member 5 is sealed by the resin case 8 and therefore linear expansion of the insulating member 5 increases and the heat distortion generated in the bonding layer 10 can be reduced in addition the thickness of the thinned base 11 is set to several mm and thus it becomes low stiffness and the heat distortion generated in the bonding layer 10 can be reduced further the thickened and high stiffened second metal base 13 and the thinned base 11 are fixed by welding friction stirring joining.