Disclosed is a femtosecond laser device. The femtosecond laser device includes a pulse oscillator configured to generate a laser pulse, a pulse width stretcher configured to stretch a width of the laser pulse, a pulse width compressor connected to the pulse width stretcher to compress the width of the laser pulse, a pulse amplifier disposed between the pulse width compressor and the pulse width stretcher to amplifier an intensity of the laser pulse, and a nonlinear pulse attenuator including an optical fiber connected between the pulse width amplifier and the pulse width stretcher and deformed to have a spiral shape, a stretched length, or a twist.