PURPOSE: To reuse the coupling and obviate exchange of a fuse plug by using non-fused metal which is deformed only at the time of over heating in the fuse plug for abnormal temperature rising of hydraulic oil in a fluid coupling.

CONSTITUTION: A fuse plug 8 is installed on the wall of an operating chamber with which hydraulic oil in a fluid coupling is filled. A hole is made in the fuse plug 8, which is covered with a bush 11, and shape memory alloy 10 is installed in a spring form. When hydraulic oil temperature rises, the shape memory allay 10 is kept in a tense condition, and hydraulic oil flows out of the hole. When hydraulic oil is cooled, the hole is covered with the bush 11. It is thus possible to make fuse replacement, unnecessary even after the fuse works due to oil temperature rising, providing reuse.