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 are disposed symmetrically to be adjacent to one another to form a shared photodiode, and a detection signal is generated when a turn on signal is simultaneously applied to transfer transistors connected to the shared photodiode.