A new method for measuring the spot size of a Gaussian laser beam is theoretically and experimentally analyzed. This method observes a boundary diffraction wave originating from a scanning straight boundary far from the laser beam axis. Theoretically, the intensity profile of the wave is verified to show the same Gaussian profile as the laser beam. Spot sizes of a 633-nm laser beam measured simultaneously by this method and the scanning knife-edge method are shown to agree well with each other over a wide range from ~0.8 to 5 microm.