A base plate having concave curved portions (rearward-convex parts) curved in a rearward direction to be convex and have a predetermined curvature, is fixed to a surface of a cooling fin while being in contact with the surface of the cooling fin at vertices of the rearward-convex parts. A stacked substrate is bonded on a front surface of the base plate, at an area opposing the rearward-convex part. A spacer is provided on a rear surface of the base plate, at a position closer than an edge of a solder layer to a perimeter of the base plate. The spacer is sandwiched between the base plate and the cooling fin when a screw for fixing the base plate to the cooling fin is tightened and the spacer has a function of suppressing deformation of the base plate.