A wide-band ultrashort-pulse optical oscillator includes: an optical amplification medium 1 that optically amplifies an incident light having a wide band or a plurality of bands so as to be converted into an oscillation light emitted from an optical resonator; an energy injection element 2 that either injects energy into the optical amplification medium so that light energy is generated, or injects light energy into the optical amplification medium; a negative dispersion element 4 that imparts a negative dispersion to a pulse light of the oscillation light; a mode locker 9 that produces a mode-locking with respect to the pulse light; a positive dispersion element 11 that imparts a positive dispersion to the pulse light; and an optical system formed so that the pulse light passes through a loop-like optical path from the optical amplification medium via the negative dispersion element, the mode locker, and the positive dispersion element, back to the optical amplification medium in at least one of the stated direction and an opposite direction. With this, it is possible to generate an ultrashort-pulse light in a wide band, with a high degree of efficiency.