The present invention measures jitter in a galvano scanner that scans a light beam on a surface to be scanned by periodically turning a reflection mirror in a reciprocating manner. Two detectors for detecting and receiving the light beam are mounted near both ends of the scan region of the light beam, a first detection signal of each of the two detectors is extracted whenever the reflection mirror is activated for one scanning, a required time for one scanning is computed from the extracted detection signals, and a variation in the required time for each scanning is measured based on the computed required time.