PURPOSE: To provide a thermally sensible steam trap which can close a large passage by enlarging the manipulating force caused by expansion and contraction of an expandable medium.

CONSTITUTION: An expandable medium is charged in an internal space defined between a wall member 13 and a diaphragm 14 which is formed from substantially semicircular pieces 14a, 14b by welding 18. The thickness of the diaphragm pieces 14a is smaller than that of the pieces 14 so that the piece 14a is more sensitive than the piece 14b. A valve element 16 having a flat valve surface 22 adapted to be seated on and lift off from a flat valve seat surface 9 which surrounds a pipe 8 is welded to the diaphragm 14 at a position 23. Thus obtained welded part 23 is positioned outside of the valve seat surface 9.