A passive mode lock fiber laser including an energy generating means. The energy generating means includes a cavity, the cavity includes a gain medium made of an optical fiber for amplifying the laser energy in the cavity, a reflect means for reflecting the laser energy, a phase fluctuation compensating means for compensating a linear phase fluctuation of the gain medium, and an output means for outputting the laser energy generated in the cavity includes a temperature adjusting mechanism for adjusting a temperature of an optical fiber portion and a piezo element position adjusting mechanism for adjusting a position in the optical axis of the reflect means by using a piezo element.