PURPOSE: To make exchange of a fuse unnecessary and to eliminate machine stop by providing a shape memory alloy, at an oil exhaust part of a fusible plug, in which an opened shape of a nozzle is stored at a high temperature and by making it return to the original closed shape at a low temperature.

CONSTITUTION: An extraction hole for oil is provided at the center of a plug 6 of a fusible plug 5, and the rotor inner circumferential side of the extraction hole is formed with a stage. A disk 8, which is divided into four portions, made of a shape memory alloy is fixed to the stage part so that the disk 8 is not opened by centrifugal pressure. When a load more than allowable transmission power is applied on a fluid coupling, a slide is generated to raise the temperature of oil to a high temperature, the disk 8 of shape memory alloy is opened, and the oil is drained out of the extraction hole. Thus, slide loss and the temperature of the oil is lowered, the disk 8 is returned to the original shape, the extraction hole is closed, and the amount of oil is returned to the normal state so as to transmit power. Therefore, it is not necessary to exchange the plug 6 and to stop the machine.