PURPOSE: To enable an open-phase protecting mechanism to be functioned speedily and suitably per load by associating a balance member acting commonly on respective poles with displacement of thermosensitive elements.

CONSTITUTION: A balance member 18 provided at its center with a vertically piercing hole with an adjusting screw 16 passed therethrough is fastened to the ground by means of a nut 17 inserted into a hexagon hole 18d. The balance member 18 is vertically displaced by the effect of the turning of the adjusting screw 16 while supporting portions 18a on both ends abut against the adjusting portions 19a of thermal elements. In the case that the thermal element on a left side is displaced in an open phase so that its reaction in an arrow direction becomes zero, reaction of the thermal element on a right side is bisected by balance action, thereby resulting in stoppage in balance. A load transferred onto the left side serves as an energy source for speeding up a removing operation. Therefore, even in the open phase, the removing operation can be speedily performed.