An apparatus for measuring the linewidth of a laser is disclosed in which the linewidth of a laser is measured in a wide range from a narrow linewidth to a wide linewidth by utilizing frequency-shifted rays based on a stimulated Brillouin scattering within an optical fiber in an easy and efficient manner. The apparatus according to the present invention includes: an optical signal generating and splitting means for generating optical signals under test, and for splitting them into two sets of signals; a frequency shifting means for amplifying the optical frequency of the optical source of the optical signal generating and splitting means, and then, irradiating the amplified signals into an optical fiber so as to shift the frequency based on a stimulated Brillouin phenomenon; a leading means for minimizing a loss of the frequency shifted rays outputted in a direction opposite to that of original laser beams of the frequency shifting means so as to leading them in a certain direction; and a spectrum analyzing means for receiving the original rays from the optical signal generating and splitting means and for receiving the frequency-shifted rays from the leading means so as to stimulate beatings and to analyze the beaten spectra. The apparatus is applied to measuring the linewidth and channel interval of a laser used in wavelength division multiplexing optical communications.