To reduce a main scan magnification error.SOLUTION: An optical scanner irradiating a scanned medium with scan light comprises: a clock generation part generating a clock signal supplied to a light-emitting element control part for controlling lighting/lighting-out of a light-emitting element; a measurement part measuring the number of pulses of clock signals generated for a period from the scan light passing by one position and then another position; and a storage part storing therein the number of reference clock pulses referred by the clock generation part. The clock generation part generates the clock signal at a cycle coincident with the number of pulses measured by the measurement part coincides with the number of the reference clock pulses. The number of the reference clock pulses is a value obtained in a state where a deviation amount of the irradiation position of the scan light in a scan direction is saturated.SELECTED DRAWING: Figure 2
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