In an imaging apparatus, a function whose value increases in time is used as a threshold electric-signal level, and an electric-signal level of a photoreceptor element is compared with the threshold electric-signal level. Even if a light received by the photoreceptor element has a low luminance, the electric-signal level of the photoreceptor element crosses the threshold electric-signal level in a short time, allowing calculation of the amount of optical energy received by the photoreceptor element. Furthermore, when an object having a region where brightness changes in time and also having a region with a low luminance is imaged, imaging interval is changed in accordance with luminance. Accordingly, information regarding how the brightness of the object is rapidly changing in time can be obtained more precisely, and an image can be output without making a projection image of the dark region completely black.