To provide a transmitter optical subassembly that provides an enhanced high-frequency response without increasing the power consumption within a CAN-type package. The transmitter optical subassembly includes a semiconductor laser diode, an auxiliary circuit, and a package for installing the semiconductor laser diode and the auxiliary circuit. The auxiliary circuit generates a transient signal synchronized with the leading edge or falling edge of a driving signal for the semiconductor laser diode in an electrode of the semiconductor laser diode for a very short period in order to boost the response speed of the semiconductor laser diode. The auxiliary circuit operated intermittently so that it is not necessary to consider heat caused by the auxiliary circuit even in case the auxiliary circuit is installed in the package.