A resistor is connected in series with a vertical cavity surface emitting laser, and DC bias electric current is supplied thereto from a DC bias electric current supplying circuit by way of an inductor. Input data is inputted from a differential input line to a voltage amplifier by way of a PECL input buffer, for voltage amplification. The amplified signal is attenuated by an attenuator, transferred to a transfer line by way of a capacitor for AC coupling, and applied to the series resistance and the vertical cavity surface emitting laser. The voltage amplifier has a peak in its frequency characteristics and is structured such that the gain, the frequency band, and the peaking characteristics can be adjusted.