An image input apparatus for inputting an image of an object and outputting the image as an electric signal, the image input apparatus comprises a stage which supports the object, a laser interferometer which measures a position of the stage, a light source which emits a pulse light, an illumination optical system which irradiates the object with an illuminating light, a sensor which converts an image-formed optical image into an electric image signal, an imaging optical system which forms an image of the object on the sensor, a synchronization control circuit which controls a light-emission interval of the light source and synchronization of the sensor on the basis of position information of the laser interferometer, a light quantity monitor which measures a quantity of light, and a light quantity correction circuit which corrects the electric image signal on the basis of an output of the light quantity monitor.