PURPOSE: To dispense with the penetrating part on a boiler body for inserting a rod and eliminate the steam leak from the inserting part by providing a piston driving device in a boiler body, and connecting a controlling skirt to the piston.

CONSTITUTION: When a steam generating amount of an electrode type electric boiler 23 increases and the pressure of a boiler body 1 goes up, a signal is outputted from a pressure switch 18 for detecting the generated pressure, each of pneumatically operated control valves 30, 31 changes its opening degree, and a pressure of water flowing into a cylinder 29 changes, in other words, that of the upper part of the water flow decreases, while that of the lower part thereof increases. Thus the piston 32 goes up due to the pressure difference and at the same time the skirt 28 for controlling rises. Accordingly the amount of generated steam decreases and the pressure within the boiler body 1 also decreases. When the amount of the generated steam decreases, the piston 32 is lowered, together with the lowering of the skirt 28, thereby increasing the amount of the generated steam. By adopting the above- mentioned mechanism, it is unnecessary to provide a penetrating part and a seal accompanied by a sliding action at a boiler body 1, and it is possible to control the steam amount automatically by a piston driving mechanism provided within a boiler body. Therefore, the reliability in preventing the steam leak from the boiler body 1 is remarkably enhanced.