We fabricated a monolithic passive mode-locked distributed Bragg reflector laser diode for the first time. An optical short-pulse train with a duration of 3.5 ps was successfully generated from the laser at a high repetition rate of over 40 GHz. The time-bandwidth product was 0.43, and it was very close to the transform-limited value of a Gaussian waveform. The highest peak power of 60 mW in an InP-based passive mode-locked laser has been achieved. This laser is promising as an optical source for a high bit rate soliton transmission system.