A method for realizing both high extraction efficiency of laser light from electron beams and femto-second ultra-short pulses in a free-electron laser, wherein the peak current of electron pulses from a superconducting or normal conducting linear accelerator serving as a driver for the free-electron laser is adjusted to be greater than a certain lower limit and electron beams are brought into perfect synchronism with light or the time of duration for which the electron pulses are repeated is made longer than the time to saturation of light oscillation (the time during which light is emitted due to the increase of optical output by an amount corresponding to the gain up to the point where the optical output does not increase any more), whereby the oscillation of light is facilitated, wherein the extraction efficiency of laser light from electron beams is increased to exceed the theoretical limit which is expressed by 1/(2 Nw)1/(4 Nw) (Nw=the number of undulator periods) of electron beam power or rendered proportional to the square of N, or the number of electrons in a micropulse, and wherein ultra-short pulses of the femto-second range are generated.