1. An image sensor with a shared photodiode, comprising: at least two unit pixels, each of which includes a photodiode, a diffusion region which gathers electrons from the photodiode, a transfer transistor which connects the photodiode with the diffusion region, and a readout circuit which reads out a signal from the diffusion region, wherein photodiodes of neighboring unit pixels from the at least two unit pixels are disposed symmetrically to be adjacent to one another to form a shared photodiode, the photodiodes of the neighboring unit pixels physically touching each other, and a detection signal is generated when a turn on signal is simultaneously applied to the transfer transistors of the neighboring unit pixlels that are connected to the shared photodiode; and a separation wall that forms an energy barrier in the shared photodiode to separate incident light within the shared photodiode.