| 摘要 |
PROBLEM TO BE SOLVED: To provide a scanning optical device capable of exactly detecting the scanning position of a plotting light beam, applicable even when a plotting objective surface is a plane and without being affected by the error of a shape of mirror even by providing the mirror for separating the optical path of a monitor light beam from the optical path of a plotting light beam. SOLUTION: A monitor light beam L2 and a plotting light beam L1 are made incident on a polygon mirror 40 at different angles so that they are separated in the subscanning direction after transmitting through a fθlens and the convergent degree of the monitor beam L2 is made larger than that of the plotting light beam L1. Consequently, the monitor light beam L2 is set so as to be converged just before a plotting objective surface 60 and a transmission scale 70 is arranged by making it coincident with the convergent position.
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