Optical semiconductor device

An optical semiconductor device comprising:

• an optical semiconductor package including
• a substrate that has a first main surface and a second main surface opposed to each other, and a through hole penetrating the first and second main surfaces,
• a semiconductor optical element disposed on the first main surface of the substrate,
• an electrode terminal disposed in the through hole of the substrate with a first end exposed on the second main surface of the substrate, secured to the substrate by a sealant, and connected to the semiconductor optical element through a signal line, and
• a cap member covering the first main surface of the substrate and sealing the semiconductor optical element;
• a flexible substrate having a flexible dielectric film and a signal transmission line on a front surface of the dielectric film, a first end of the signal transmission line being electrically connected to the electrode terminal of the optical semiconductor package on a second main surface side of the substrate of the optical semiconductor package;
• and
• impedance matching means matching impedance of the signal transmission line of the flexible substrate and impedance of the electrode terminal of the optical semiconductor package at the second main surface side of the substrate.