A plasma reactor includes a honeycomb electrode in which a plurality of cells that function as gas passages are partitioned by a partition wall, and a discharge electrode. The honeycomb electrode includes a first gas circulation section that allows a first gas to pass through, and a second gas circulation section that allows a second gas to pass through. The plasma reactor causes the first gas introduced into the first gas circulation section of the honeycomb electrode through the space between the electrodes to undergo a reaction while causing a plasma discharge between the honeycomb electrode and the discharge electrode, and allows the second gas to be introduced into the second gas circulation section of the honeycomb electrode to transfer heat of the second gas to the first gas circulation section to promote the reaction of the first gas.