A temperature control module and a support block are mounted on a metal stem. A dielectric substrate is mounted on a side surface of the support block. A support block is mounted on a cooling surface of the temperature control module. A dielectric substrate is mounted on a side surface of the support block. A semiconductor optical modulation element is mounted on the dielectric substrate. A lead pin and a signal line are connected through a bonding wire. The signal line and a signal conductor are connected through a bonding wire. The signal conductor and the semiconductor optical modulation element are connected through a bonding wire.