PURPOSE: To provide a thermally sensible steam trap which can increase an operation force caused by expansion and contraction of an expandable medium so as to open and close a large passage.

CONSTITUTION: Expandable medium 16 is charged in an internal space defined between a wall member 13 and a diaphragm 14. Further, expandable medium 17 is charged in an internal space defined between the wall member 13 and a diaphragm 15. The thickness of the diaphragm 14 is thin so that it is sensitive in comparison with the diaphragm 15. A valve element 18 having a flat valve surface 26 which can be seated onto and lifted off from a valve seat surface 9 surrounding a pipe 18 is welded to the diaphragms 14, 15 (at positions 27, 28). The welded parts 27, 28 are located outside of the valve seat surface 9.