According to one embodiment, a photodetector includes a sensor unit including at least one photosensor, a liquid crystal element including a first substrate includes a plurality of first control electrodes, a second substrate including a second control electrode, and a liquid crystal layer held between the first substrate and the second substrate, a liquid crystal controller which controls a voltage to be applied to the liquid crystal layer for forming a first lens that is opposed to the photosensor in the liquid crystal layer, and a sensor controller which controls the sensor unit.