A unit pixel of an image sensor having a three-dimensional structure includes a first chip and a second chip which are stacked, one of the first chip and the second chip having a photodiode, and the other of the first chip and the second chip having a circuit for receiving information from the photodiode and outputting received information. The first chip includes a first pad which is projectedly disposed on an upper surface of the first chip in such a way as to define a concavo-convex structure, and the second chip includes a second pad which is depressedly disposed on an upper surface of the second chip in such a way as to define a concavo-convex structure corresponding to the concavo-convex structure of the first chip. The first chip and the second chip are mated with each other through bonding of the first pad and the second pad.