An activated gas injector includes a flow passage defining member partitioned into a gas activation passage and a gas introduction passage by a partition wall; a gas introduction port through which a process gas is introduced into the gas introduction passage; a pair of electrodes to be supplied with electrical power to activate the process gas, wherein the electrodes extend along the partition wall in the gas activation passage; through-holes formed in the partition wall and arranged along a longitudinal direction of the electrodes, wherein the through-holes allow the process gas to flow from the gas introduction passage to the gas activation passage; and gas ejection holes provided in the gas activation passage along the longitudinal direction of the electrodes, wherein the gas ejection holes allow the process gas activated in the gas activation passage to be ejected therefrom.