Expansion device eliminating the slack of the expanded piece of the expanded workpiece unit even when the expansion pieces of a plurality of workpiece units are continuously expanded

An expansion device is provided, which can eliminate the slack of the expanded piece of the expanded workpiece unit to prevent chip damage and suppress the next expanded workpiece unit from poor cooling even when the expansion piece of a plurality of workpiece units are continuously expanded. The expansion device (1) has: a frame holding unit (20) that holds the ring frame of the workpiece unit; a cooling table (30) that includes an abutting surface (31) and abuts against the expansion piece of the workpiece unit that is held by the frame holding unit (20); an expansion unit (40) which expands the expansion piece; a heating unit (50) which has a warm air jetting part (52) that sprays warm air to the expansion piece between the outer peripheral side of the workpiece and the inner periphery of the ring frame; and a curved plate (90) which is arranged between the workpiece and the warm air jetting part (52), and has an air outlet that sprays air toward the workpiece to prevent the intrusion of warm air sprayed b