An activated gas reaction apparatus which comprises an activation chamber; feeding means for conducting feed gas into the activation chamber; microwave power-generating means for activating raw gas received in the activation chamber; and a reaction chamber provided apart from the activation chamber for reaction of activated gas, the activated gas reaction apparatus so constructed as to satisfy the following formula:2.pi.fpl .gtoreq. 4 .times. 10.sup.9wheref = frequency (Hz) of microwavesp = gas pressure (torr) in the activation chamberL = length (cm) of the activation chamber extending in the direction in which the electromagnetic field provided by the microwave source is applied.