To provide a radial beam-scanning method which includes the compensation for the displacement of a reflected beam introduced by the reflector by changing a light path in a part with intervals on the circumference of the beam on the upstream side of the reflector in synchronization with the rotation of the reflector and starts scanning by the incidence of a beam on a rotary reflector.

A radial beam scanner includes a reflector 10 which can rotate so as to cause a scanning motion of an incident radial beam, and a compensating device 4 on which the radial beam falls before it reaches the rotary reflector 10. The compensating device 4 compensates the distortion of a reflected beam introduced by the reflector by changing a light path in a part with intervals on the circumference of the beam on the upstream side of the reflector in synchronization with the rotation of the reflector.