A Cossy 5-exo-dig photocyclization of organohalides (X = Br, I) onto a C-C triple bond was studied using a flow photomicroreactor, which proceeded in a minute order of residence time. A deuterium labeling study supported the nonchain radical mechanism proposed by the Cossy group, in which a hydrogen source originates from a triethylamine cation radical. Scalable flow synthesis using a larger volume of flow reactor was also successful, providing 4 g of the product in high yield.