Abstract: An optical signal is output from an optical semiconductor element according to a high frequency signal. The optical semiconductor element is surrounded by a metallic package having a package base, a package cover and a seal ring so as to separate the optical semiconductor element from the outside air. An electromagnetic wave absorptive element is arranged on an inner surface of the metallic package to attenuate the high frequency signal leaking into the cavity of the metallic package, and the electromagnetic wave absorptive element is covered with a seal element so as to hermetically seal the electromagnetic wave absorptive element from the cavity. The seal element allows the high frequency signal to be transmitted through the seal element. The high frequency signal leaking into the cavity is transmitted through the seal element and is converted into heat energy in the electromagnetic wave absorptive element.