A photodetector according to an embodiment includes: a semiconductor substrate with a first and second faces; a groove formed on the second face; pixels disposed to the semiconductor substrate, each pixel including: light detection cells disposed on the first face, each light detection cell having a first and second terminals, each light detection cell being surrounded by the groove; a first wiring line disposed on the first face to connect to the first terminal of each of the detection cells; a first opening formed in the second face and penetrating the semiconductor substrate; a first insulating film covering the second face, a side face of the first opening, and a side face and a bottom of the groove; a second opening formed in the first insulating film; a first and second electrodes disposed in the first and second openings respectively; and a light blocking material filled to the groove.