A reactor causing reaction of a reactant includes a plurality of substrates provided with top and bottom substrates having provided therein recessed portions for forming closely sealed regions and a plurality of intermediate substrates having provided therein at least openings that communicate with each other, a reactor main body portion including a reaction unit at which the intermediate substrates are formed to be laminated and joined with each other and a reaction flow channel reactant flows formed therein, and an envelope portion which houses the reactor main body portion therein except one end side thereof, via a closely sealed space formed by sandwiching the laminated intermediate substrates between the top and the bottom substrates and by communicating the opening and the closely sealed region on the substrates, and including a support portion which supports the reactor main body portion via the one end side of the reactor main body portion.