A semiconductor device includes a semiconductor element having a rectangular two-dimensional geometry and serving as a heat source, a first heat sink section including the semiconductor element mounted thereon, and a second heat sink section joined to an opposite side of the first heat sink section that includes the semiconductor element. A relation among directional components of thermal conductivity is K1yy≧K1xx>K1zz, where directional components of a three-dimensional thermal conductivity of the heat sink section in X, Y, and Z directions are determined as Kxx, Kyy, and Kzz. A relation among directional components of a thermal conductivity of the second heat sink section is K2zz≧K2yy>K2xx or K2yy≧K2zz>K2xx, where the directional components of the thermal conductivity of the second heat sink section in X, Y, and X directions are determined as K2xx, K2yy, and K2zz.