An image sensor repeatedly receives a light from an object through a lens and generates images of the object. A differential image generating unit generates a differential image from at least two images. A detector detects a change cycle or an intensity of an external light. An evaluation unit evaluates an influence of the external light and derives an optimal timing in which a minimum influence of the external light is attained. A controller controls a timing of receiving the light of the image sensor, depending upon the optimal timing derived by the evaluation unit, and repeatedly synchronizes a term of two timings with the change cycle of the external light by shifting the term into the change cycle by a unit of predetermined phase difference.