A signal processing device comprising: a comparison circuit configured to perform a first comparison of a noise signal with a first reference voltage and a second reference voltage, wherein the first reference voltage is scanned in steps of a first gradation precision, the second reference voltage is scanned in steps of a second gradation precision, and the first gradation precision is higher than the second gradation precision, wherein the first comparison is performed by sequentially comparing the first and second reference voltage with the noise signal, a second comparison of an image data signal with a determination voltage, and a third comparison of the image data signal with a selected reference voltage; a time measurement circuit configured to count, in the first comparison, first and second sequential periods of time from when the comparison circuit starts comparing the noise signal respectively with the first and second reference voltages to when a value of a comparison result changes and output respective first and second count values as digital data of the noise signal; and a selection circuit (163, 263) configured to select the selected reference voltage from the first and second reference voltages based on the second comparison, wherein the noise signal and the image data signal are detected by an image sensor. a comparison circuit configured to perform a first comparison of a noise signal with a first reference voltage and a second reference voltage, wherein the first reference voltage is scanned in steps of a first gradation precision, the second reference voltage is scanned in steps of a second gradation precision, and the first gradation precision is higher than the second gradation precision, wherein the first comparison is performed by sequentially comparing the first and second reference voltage with the noise signal, a second comparison of an image data signal with a determination voltage, and a third comparison of the image data signal with a selected reference voltage; a first comparison of a noise signal with a first reference voltage and a second reference voltage, wherein the first reference voltage is scanned in steps of a first gradation precision, the second reference voltage is scanned in steps of a second gradation precision, and the first gradation precision is higher than the second gradation precision, wherein the first comparison is performed by sequentially comparing the first and second reference voltage with the noise signal, a second comparison of an image data signal with a determination voltage, and a third comparison of the image data signal with a selected reference voltage; a time measurement circuit configured to count, in the first comparison, first and second sequential periods of time from when the comparison circuit starts comparing the noise signal respectively with the first and second reference voltages to when a value of a comparison result changes and output respective first and second count values as digital data of the noise signal; and a selection circuit (163, 263) configured to select the selected reference voltage from the first and second reference voltages based on the second comparison, wherein the noise signal and the image data signal are detected by an image sensor.