A nuclear reactor includes a reflector and a flow path. The reflector reflects neutrons, contains graphite and a moderator having a smaller moderating power than the graphite, and is sectioned into plural parts along a direction of flow of fuel pebbles. The flow path is surrounded by the reflector, and the fuel pebbles flow through the flow path and undergo nuclear reaction to generate power. Volume ratio of the graphite to the moderator having a smaller moderating power than the graphite in each part of the reflector is determined based on a power distribution in the reactor core in the direction of flow of the fuel pebbles.