Disclosed is a scale removing device for removing scale adhering to the interior of a shell-and-tube-heat-exchanger-type steam generator. The scale removing device is provided with a flexible lance (58) holding a high-pressure water hose (91) and movable with respect to the tube plate, the tube support plates, and the heat-transfer tubes, and a cleaning head (60) mounted to the forward end of the flexible lance. Formed inside the cleaning head is a cavitation generating nozzle hole (99a, 99b) communicating with the high-pressure water hose. Bubbles are generated at the time of cleaning, and the impact pressure generated when the bubbles collapse is propagated to remove scale over a wide range.