To provide a small-size image sensor and electromagnetic radiation imaging device which can obtain a good image without relying on the condition of an object, the image sensor including a plurality of pixel units arranged two-dimensionally, wherein each of the plurality of pixel units includes: a first photodiode and a second photodiode; a readout circuit which reads a signal generated by the first photodiode and a signal generated by the second photodiode, and outputs the read signals, the readout circuit being connected to the first photodiode and the second photodiode; and a difference circuit which outputs a difference signal corresponding to a difference between the signal read from the first photodiode and the signal read from the second photodiode, the difference circuit being connected to the readout circuit.