An optical system of a laser scanner comprising: a light projecting system for projecting a distance measuring light, a scanning mirror for rotatably irradiating the distance measuring light from the light projecting system around a single axis and for making a reflected distance measuring light from an object to be measured enter a light receiving system, a transmission window for accommodating the scanning mirror and through which the distance measuring light and the reflected distance measuring light are transmitted, and a correction optical component for offsetting an optical action of the transmission window, which is provided at least in a middle of an irradiating optical path of the distance measuring light.