To provide a thermally reactive steam trap for allowing the washing of foreign matters deposited on the inner face on the valve chest side of a valve hole without lowering the drain flow amount of condensate while maintaining a steam trap function to drain condensate without leaking steam when an expansion rate is a predetermined value or higher because of the transformation of an expansive/contractive medium.

The thermally reactive steam trap includes a body 1 and a cover 2 to form an inlet 4, a valve chest 3 and an outlet 5, a valve seat member 7 having the valve hole 10 for communicating the valve chest 3 with the outlet side, and arranged between the valve chest 3 and the outlet 5, and a temperature control element 12 using the expansive/contractive medium 18, and arranged in the valve chest 3. The valve seat member 7 is formed spherical, and the valve hole 10 is T-shaped. The spherical valve seat member 7 having the T-shaped valve hole 10 whose long passage has one end 10c formed in an orifice passage is arranged on the body 1 in a rotation operable manner. A bypass line 8 communicating the inlet 4 with the spherical valve seat member 7 is formed in the body 1.