A GePD having uniform sensitivity in C and L bands is provided and a photodetector in which deterioration of a common-mode rejection ratio is curbed is provided. A photodetector according to an embodiment includes one or a plurality of sets of two photodiodes to which a differential signal is input, a monitor connected to each of the two photodiodes and configured to measure a photocurrent, and a voltage supply configured to control a voltage applied to each of the two photodiodes, and the voltage supply controls the voltage applied to each of the two photodiodes so that the photocurrent measured by the monitor connected to one of the two photodiodes is equal to the photocurrent measured by the monitor connected to another one of the two photodiodes.