An image sensor includes a first pixel outputting a first pixel signal, a second pixel outputting a second pixel signal, a ramp signal generator, and a comparator. The first pixel includes first transfer transistor, a first floating diffusion node, and a first gain control transistor. The second pixel includes a second transfer transistor and a second floating diffusion node. The comparator compares the first and second pixel signals. The first pixel operates in an HCG mode to output the first pixel signal as a first HCG pixel signal in a first period and operates in an LCG mode to output the second pixel signal as a first LCG pixel signal in a second period. A reset level of the second floating diffusion node of the second pixel is provided to the comparator through the column line in an interim reset period between the first period and the second period.