A drive current is generated by mixing a pulse signal from a pulse generator and a DC current from a DC current power supply using a T circuit and injected into a nitride semiconductor laser having a horizontal light-confinement ridge structure. The horizontal light-confinement coefficient of the nitride semiconductor laser is between 85% and 99%. The time when the current waveform of the drive current is continuously over the threshold current of the nitride semiconductor laser ranges from 5 nsec to 1,000 nsec.