A reactor to reform fuel includes a low temperature reaction unit, a high temperature reaction unit provided separately from the low temperature reaction unit, and a communicating tube via which the low temperature reaction unit and the high temperature reaction unit communicate with each other. At least one of the low temperature reaction unit and the high temperature reaction unit has a first surface, and the other of the low temperature reaction unit and the high temperature reaction unit has a second surface which is opposed to the first surface. The first surface includes a concave surface and a projecting surface which projects towards the second surface, wherein a length between the concave surface and the second surface is longer than a length between the projecting surface and the second surface. And the communicating tube is provided between the concave surface and the second surface.