PURPOSE: To make image pickup signal level optimum and to simplify processing of position-detecting signal, by appropriately controlling a semiconductor chip lighting intensity or scanning speed of an image pickup device which scanningly detects image of the chip.

CONSTITUTION: An image signal of a semiconductor chip 1 from an image pickup device 4 is put into a calculator 7 through an AC-DC converters 6 and accumulated in a memory 8. The calculator 7 seeks an average value of detection level from picture element level and feeds this back to an image pickup control device 5. The image pickup control device 5 controls the image scanning speed in the image pickup device 4 on the basis of the feedback information or energizes a light source driving device 9 to set lighting intensity of a light source 3 variably, etc. so that an average level calculated by the calculator 7 becomes lower than the saturation level of a photoelectric detector in the image pickup device 4.