Each unit pixel includes a photodiode, a reading selection transistor, a reading transistor, an amplifying transistor, a reset transistor, and a horizontal selection transistor, and thus a MOS image sensor of a dot-sequential reading 5-Tr type is formed. The reading selection transistor and the reading transistor are formed with a two-layer gate structure, and gate potential of the reading selection transistor and the reading transistor is set to a negative potential. Thereby, a lower layer of a gate region of the reading transistor and the reading selection transistor is controlled to a negative potential. Thus, depletion in the lower layer region is suppressed to reduce leakage current.