To provide a self-closing valve device of a flash valve or the like that can positively prevent freezing of toilet bowl trap water by making water flow out automatically and continuously when air temperature is low.

When the air temperature is higher than a critical point of a shape memory spring 50, the shape memory spring 50 is contracted as shown in a figure, and a valve is operated in the same way as a normal flush valve. When the air temperature is lower than the critical point of the shape memory spring 50, the shape memory spring 50 is elongated, and a push rod 30 advances without pressing a push button 280 so as to push a valve stem 26a to make water in a pilot water chamber B flow out of an outflow port 14. Water in an inflow chamber A therefore flows out to an outflow chamber C continuously by a small quantity. Toilet bowl trap water is therefore prevented from being frozen.