To extend the service life of a diaphragm of a heat responding steam trap.

A first diaphragm 14 is formed in a waveform having a plurality of annular protrusions 21 in a concentric circle shape. A second diaphragm 18 is also formed in a waveform having a plurality of annular protrusions 22 in a concentric circle shape along the annular protrusions 21 of the first diaphragm 14. An upper surface of a lower wall member 19 to which a lower surface of the second diaphragm 18 abuts when an expansion medium 15 is expanded is also formed in a shape having a wave-shaped part 23 in a concentric circle shape which is the same with or approximates a lower surface of the second diaphragm 18 so as to follow the waveform of the lower surface of the second diaphragm 18. The lower surface of an upper wall member 11 to which the upper surface of the first diaphragm 14 abuts when the expansion medium 15 is contracted is also formed in a shape having a wave-shaped part 24 in a concentric circle shape which is the same with or approximates the upper surface of the first diaphragm 14 so as to follow the waveform of the upper surface of the first diaphragm 14.