PURPOSE: To achieve economic frost prevention by using a spring made of shape memory alloy having Martensitic transformation temperature near the freezing point of water and opening a valve at a temperature below the freezing point of water to fluidize the water in a pipe.

CONSTITUTION: When the ambient temperature is below the freezing point, deflection of an ordinary metal spring 1 is set at L1 to generate a force P1. Next, deflection of a spring 2 made of shape memory alloy which has Martensitic transformation temperature near the freezing point of water is set at L to ganerate a force P and a condition P >P is set to open the tip of a valve 4, following which when the ambient temperature becomes above the freezing point, the deflection of the spring 2 changes to L based on its shape memory property. Further a deflection variation equal to the deflection variation of the spring 2 L-L is generated in the spring 1, so that a force generated in the spring l changes from P1 to P2. And the force of the spring 2 becomes P, and a condition P >P is set to close the tip of the valve 4. As a result, frost damage can be easily prevented without using the energy of electricity, gas etc.