An imaging device includes an imaging element that takes an image of a target object and output an image signal corresponding to the image; a storage unit that stores therein a zero-point adjustment reference value; a brightness detecting unit that detects a brightness of the target object; and a reference-value changing unit that changes the zero-point adjustment reference value when the brightness detected by the brightness detecting unit is equal to or lower than a threshold. A zero point in a level of the image signal output from the imaging element is adjusted by using the zero-point adjustment reference value stored in the storage unit.