A photodiode comprises a support substrate, an insulating layer formed over the support substrate, a silicon semiconductor layer formed over the insulating layer and having a device forming area and device isolation areas which surround the device forming area, a device isolation layer formed in the device isolation areas, a P+ diffusion layer formed in the device forming area close to one edge lying inside the device isolation layer by diffusing a P-type impurity in a high concentration, an N+ diffusion layer spaced away from the P+ diffusion layer and formed in the device forming area close to the other edge opposite to the one edge of the device isolation layer by diffusing an N-type impurity in a high concentration, a low concentration diffusion layer formed in the device forming area located between the P+ diffusion layer and the N+ diffusion layer by diffusing an impurity of the same type as either one of the P+ diffusion layer and the N+ diffusion layer in a low concentration, and silicide layers respectively formed above the P+ diffusion layer and the N+ diffusion layer with being spaced away from a boundary between the low concentration diffusion layer and the P+ diffusion layer and a boundary between the low concentration diffusion layer and the N+ diffusion layer.