To reduce the possibility of a bimetal ring being struck by a valve disc and that of an outer ring valve seat being worn.

The valve disc 11 is positioned within a transformer chamber 4 constructed of inner and outer ring valve seats 6, 7, a protective seat 16 and a lid member 5 and an ended bimetal ring 13 is positioned on a slope part 12 between the outer ring valve seat 7 and the protective seat 16. An annular plate 14 is positioned between the valve disc and the bimetal ring. The valve disc 11 has an outside diameter greater than the inside diameter of the protective seat 16 and has twelve through holes 15 on its outer periphery which communicate the top to the bottom of the valve disc 11. Each of the through holes 15 is partially located between the inner and outer peripheries of the outer ring valve seat 7. Steam passing at high speed between the outer ring valve seat 7 and the valve disc 11 can enter the transformer chamber 4 from a gap 16 between the projections 15 of the valve disc 11, so the amount of steam passing between the annular plate 14 and the valve disc 11 is reduced enough so that the annular plate 14 is less likely to be sucked upwards and the bimetal ring 13 less likely to be struck by the valve disc 11 via the annular plate 14.