An optical beam scanner, for reading a bar-code, which comprises a housing (10) accommodating therein a beam generator (1) for projecting a laser beam (34) along a fixed axis, a rotary mirror (30, 40) rotatable about its axis (32) and having a plurality of reflection surfaces (30a, 30b) arranged in a polygonal manner on the periphery of a body thereof, which surfaces receive and reflect the laser beam (34) projected from the beam generator (1), and a unit for driving the rotary mirror (30) to rotate at a predetermined speed during the operation. The housing is provided with a window (21) on an upper wall thereof, through which passes the laser beam (35) reflected from the respective reflection surfaces (30a, 30b) of the rotary mirror (30) for scanning a bar-code disposed above the window (21), and each of the reflection surfaces (30a, 30b) of the rotary mirror (30) is formed by a part of a circular cylindrical surface cut out along a height thereof in an elongated rectangular shape and disposed so that a ridgeline (33) is transverse to the axis (32) of the rotary mirror (30). A direction of a normal vector to the reflection surface (30a, 30b) is smoothly varied in one direction along a scanning line (X) on the reflection surface (30a, 30b) traced by sequential points of incidence of the laser beam (34) across the reflection surface (30a, 30b) when the rotary mirror (30) is rotated, whereby the reflected beam (35) traces various linear scanning patterns (A, B), each corresponding to the respective reflection surface (30a, 30b), on a plane of the window (21) above which the bar-code is scanned.