An active energy radiation unit comprising: an active energy radiation part which radiates active energy rays onto a target object disposed in an active energy radiation region extending in a first direction; and a main gas supply mechanism which extends in the first direction, is disposed to be adjacent to the active energy radiation part in a second direction intersecting the first direction, and ejects an inert gas for forming an inactive region including the active energy radiation region between the target object and the active energy radiation part, wherein the main gas supply mechanism includes: a receiving part which receives the inert gas, a first ejection port which is provided between the receiving part and the active energy radiation part in the second direction and is closer to the target object than the receiving part, a second ejection port which is provided between the receiving part and the first ejection port in the second direction, and a flow path part and a pair of slope surfaces, wherein the flow path part is a region surrounded by the pair of slope surfaces extending toward a downstream side in the second direction, receives the inert gas, and ejects the inert gas from the first ejection port and the second ejection port, wherein the receiving part is an inlet of the flow path part, the first ejection port is an outlet of the flow path part, the inert gas ejected from the first ejection port forms a first flow that flows in a direction of the flow path part, and the inert gas ejected from the second ejection port forms a second flow that flows toward the target object. an active energy radiation part which radiates active energy rays onto a target object disposed in an active energy radiation region extending in a first direction; and a main gas supply mechanism which extends in the first direction, is disposed to be adjacent to the active energy radiation part in a second direction intersecting the first direction, and ejects an inert gas for forming an inactive region including the active energy radiation region between the target object and the active energy radiation part, wherein the main gas supply mechanism includes: a receiving part which receives the inert gas, a first ejection port which is provided between the receiving part and the active energy radiation part in the second direction and is closer to the target object than the receiving part, a second ejection port which is provided between the receiving part and the first ejection port in the second direction, and a flow path part and a pair of slope surfaces, wherein the flow path part is a region surrounded by the pair of slope surfaces extending toward a downstream side in the second direction, receives the inert gas, and ejects the inert gas from the first ejection port and the second ejection port, wherein the receiving part is an inlet of the flow path part, the first ejection port is an outlet of the flow path part, the inert gas ejected from the first ejection port forms a first flow that flows in a direction of the flow path part, and the inert gas ejected from the second ejection port forms a second flow that flows toward the target object. a receiving part which receives the inert gas, a first ejection port which is provided between the receiving part and the active energy radiation part in the second direction and is closer to the target object than the receiving part, a second ejection port which is provided between the receiving part and the first ejection port in the second direction, and a flow path part and a pair of slope surfaces, wherein the flow path part is a region surrounded by the pair of slope surfaces extending toward a downstream side in the second direction, receives the inert gas, and ejects the inert gas from the first ejection port and the second ejection port, wherein the receiving part is an inlet of the flow path part, the first ejection port is an outlet of the flow path part, the inert gas ejected from the first ejection port forms a first flow that flows in a direction of the flow path part, and the inert gas ejected from the second ejection port forms a second flow that flows toward the target object.