A 3D image sensor includes a depth pixel that includes; a photo detector generating photo-charge, first and second floating diffusion regions, a first transfer transistor transferring photo-charge to the first floating diffusion region during a first transfer period in response to a first transfer gate signal, a second transfer transistor transferring photo-charge to the second floating diffusion region during a second transfer period in response to a second transfer gate signal, and an overflow transistor that discharges surplus photo-charge in response to a drive gate signal. Control logic unit controlling operation of the depth pixel includes a first logic element providing the first transfer gate signal, a second logic element providing the second transfer gate signal, and another logic element providing the drive gate signal to the overflow transistor when the first transfer period overlaps, at least in part, the second transfer period.