A semiconductor device 1 has a metallic base substrate 2 for heat-dissipating, a wiring board 3, a MOSFET 4 as a semiconductor element, externally leading terminals 5A, 5B, 5C, a casing 6 formed of a synthetic resin, a fixing resin 7, and a gel-like resin layer 8. On the metallic base substrate 2, the casing 6 is disposed to surround one ends of the externally leading terminals 5A, 5B, 5C and the MOSFET 4. The other ends of the externally leading terminals 5A, 5B, 5C are externally protruded from the casing 6, and terminal body portions 51A, 51B, 51C for coupling them are inserted in through hole portions 61A, 61B, 61c of the casing 6. The terminal body portions 51A, 51B, 51C and the through hole portions 61A, 61B, 61c are fixed with the fixing resin 7, and the gel-like resin layer 8 is formed at the lower part of the casing 6. A space 9 is formed between the gel-like resin layer 8 and the casing 6.