A light amplifier comprising: a light amplifying fiber for amplifying seed light with excitation light; a seed light source for pulsing the seed light for a plurality of times during an emission period as a plurality of groups of pulses each group including a plurality of pulses; an excitation light source for, on a repeated cycle, generating the excitation light with a first level of power during a non-emission period, which is immediately before the emission period, and generating the excitation light with a second level of power, which is higher than the first level, during the emission period; a detector for detecting a peak power of a group of output light pulses which are output from the light amplifying fiber; wherein the detector is adapted to detect the peak power of a first group of output light pulses and the peak power of a final group of output light pulses output from the light amplifying fiber during the emission period; and a control unit for controlling the power of the excitation light of the non-emission period by feedback control based on the detected values from the detector to cause the peak power of the first group of output light pulses which are generated during the emission period to be the same as the peak power of the final group of output light pulses, wherein the control unit is adapted to control the first level of power based on a comparison of the detected peak power value of the first group of output pulses and the detected peak power value of the final group of output pulses from the detector, and wherein, the control unit is further configured to, during a starting period of the light amplifier during which the control unit is configured to prevent the output light pulses from being output to the outside of the light amplifier, control the power of the excitation light during the non-emission period by said feedback control for each of a plurality of predetermined conditions of the output light pulse, said predetermined conditions at least including a repetition frequency of a group of pulses and the number of pulses in a group of pulses, and to obtain data in association with the predetermined conditions on the first level of power of the excitation light for causing the power of the first group of output light pulses to be the same as the power of the final group of output light pulses, and wherein the light amplifier further comprises a storage unit for storing the data in association with the predetermined conditions of the output light pulses. a light amplifying fiber for amplifying seed light with excitation light; a seed light source for pulsing the seed light for a plurality of times during an emission period as a plurality of groups of pulses each group including a plurality of pulses; an excitation light source for, on a repeated cycle, generating the excitation light with a first level of power during a non-emission period, which is immediately before the emission period, and generating the excitation light with a second level of power, which is higher than the first level, during the emission period; a detector for detecting a peak power of a group of output light pulses which are output from the light amplifying fiber; wherein the detector is adapted to detect the peak power of a first group of output light pulses and the peak power of a final group of output light pulses output from the light amplifying fiber during the emission period; and a control unit for controlling the power of the excitation light of the non-emission period by feedback control based on the detected values from the detector to cause the peak power of the first group of output light pulses which are generated during the emission period to be the same as the peak power of the final group of output light pulses, wherein the control unit is adapted to control the first level of power based on a comparison of the detected peak power value of the first group of output pulses and the detected peak power value of the final group of output pulses from the detector, and wherein, the control unit is further configured to, during a starting period of the light amplifier during which the control unit is configured to prevent the output light pulses from being output to the outside of the light amplifier, control the power of the excitation light during the non-emission period by said feedback control for each of a plurality of predetermined conditions of the output light pulse, said predetermined conditions at least including a repetition frequency of a group of pulses and the number of pulses in a group of pulses, and to obtain data in association with the predetermined conditions on the first level of power of the excitation light for causing the power of the first group of output light pulses to be the same as the power of the final group of output light pulses, and wherein the light amplifier further comprises a storage unit for storing the data in association with the predetermined conditions of the output light pulses.