A chlorine dioxide gas generator (100) is provided which capable of efficiently generating and gasifying a chlorine dioxide solution within a short period of time without accidental leakage, facilitating fumigation, and facilitating washing of the interior of the generator within a short period of time. The chlorine dioxide gas generator (100) includes a separation tank (20) that separates a chlorine dioxide gas (30b) from a chlorine dioxide solution (30a) generated in the reactor (10), the separation tank (20) including a separation cylinder (21), downwardly convex trays (22), upwardly convex tray covers (23), nubs (24) packed between the trays (22) and tray covers (23), a liquid supply pipe (25) communicating with an upper portion of the separation cylinder (21), and an air-mixture cylinder (27) that forms an air-mixture space (27a) around the separation cylinder (21) on a waste liquid chamber (26).
