To provide a microwave reactor which can prevent unnecessary heat radiation from a catalyst filled portion and promote catalytic reaction efficiently with energy saving even in a microwave irradiator having an air cooling fan.

In the microwave reactor, microwaves are radiated to the catalyst filled portion in a reaction tube to perform the catalytic reaction of a gas fluid introduced into the catalyst filled portion in the microwave irradiator having the air cooling fan, the periphery of the reaction tube corresponding to a position of the catalyst filled portion is covered with a microwave permeable heat insulating material having a specific dielectric constant of 1.0-5.0.