There is provided a gas cluster processing device for performing a predetermined process on a workpiece by irradiating the workpiece with a gas cluster, including: a processing container in which the workpiece is disposed; a gas supply part configured to supply a gas for generating the gas cluster; a flow rate controller configured to control a flow rate of the gas supplied from the gas supply part; a cluster nozzle configured to receive the gas for generating the gas cluster at a predetermined supply pressure, spray the gas into the processing container maintained in a vacuum state, and convert the gas into the gas cluster through an adiabatic expansion; and a pressure control part provided in a pipe between the flow rate controller and the cluster nozzle and including a back pressure controller configured to control a supply pressure of the gas for generating the gas cluster.