Abstract: A fiber laser includes: a seed light source; a preamplifier unit including a first optical fiber doped with a first rare earth element and amplifying a pulse seed light; a main amplifier unit including a second optical fiber doped with a second rare earth element and the first rare earth element and further amplifying light amplified by the preamplifier unit; and a buffer optical fiber provided between the preamplifier unit and the main amplifier unit, and doped with the second rare earth element. Amplified spontaneous emission light emitted by energy stored in the second rare earth element and the second excitation light not absorbed into the first and second rare earth elements of the second optical fiber but remaining are absorbed into the second rare earth element of the buffer optical fiber.