According to an embodiment, an image sensor using photon counting includes: a plurality of sub-pixels arranged in one pixel; pulse generators which are respectively connected to the plurality of sub-pixels, and are configured to generate pulse signals based on electric signals generated based on photons incident on the plurality of sub-pixels; a synchronizer and serializer configured to synchronize and serialize pulse signals respectively output from the pulse generators; and a counter configured to count and output a number of the pulse signals that are synchronized and serialized.