PURPOSE:To stably and automatically control the out-of-focus at a heating point for automation by using two light fiber element wires for detection of light output in a light beam heater for the fiber. CONSTITUTION:The lights 22a, 22b, 23a, 23b radiated from each exiting end 11a', 11b', 12a', 12b' for the light detecting fiber irradiate the corresponding photodetectors 24a, 24b, 25a, 25b. A comparator 26 compares output signals from the photodetectors 24a and 24b, and when the output signal from the photodetector 24a is larger than the output signal from the photodetector 24b, a driving motor for a relative position control mechanism in the X-axis direction is driven as if the focus moves in the direction of an arrow A, therefore, a signal is sent to the control part 30. The control part 30 drives the driving motor until the signal from the comparator 26 comes under a fixed value.
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