To provide a thermally reactive steam trap which never causes leak of steam even when foreign matter is bitten between a seal surface of a valve seat member and a valve member.

The trap includes a gasket 16 as a seal member which seals a gap between the inner circumferential wall of a lower casing 2 and the outer circumference of a temperature control element 9. The temperature control element 9 includes a fluid passing hole 14 formed to extend through a wall member 10, a diaphragm member 11 and a valve member 13, and the valve seat member 7 includes a seal surface 18 which closes the fluid passing hole 14 when the valve member 13 is seated in the valve seat member 7. The trap further includes a bimetal 15 as a temperature reactive member which deforms at a predetermined high temperature higher than the expanding/contracting temperature of an expansion medium 12 and lower than the saturation temperature thereof to open the fluid passing hole 14 at a temperature lower than the predetermined high temperature and close the fluid passing hole 14 at a temperature higher than the predetermined high temperature.