To provide a disk type steam trap capable of preventing valve opening delay of a valve disk.

A pressure changing chamber is formed of a body 1 and a lid member 10 in the inside, an inlet 2 and the pressure changing chamber are communicated with each other by opening a jetting opening 4 in the center of the pressure changing chamber side of the body 1, an annular groove 5 is arranged in the outer periphery of the jetting opening 4, thereby an inner ring valve seat 7 is formed between the jetting opening 4 and the annular groove 5 and an outer ring valve seat 8 concentric and coplanar with the inner valve seat 7 is formed in the outer periphery of the annular groove 5, the pressure changing chamber is communicated with an outlet 3 by opening a discharge opening 6 from the annular groove 5, a disk-shape valve disk 17 with a diameter larger than that of the outer ring valve seat 8 spaced apart and seated on a valve seat surface comprising the inner ring valve seat 7 and the outer ring valve seat 8 is arranged in the pressure changing chamber, the pressure changing chamber is divided into a first pressure changing chamber 12 where the valve disk 17 is arranged and a second pressure changing chamber 13 as a heat insulation chamber outside of the same by a bimetal 11 as a temperature sensitive responding element, a communicating passage 14 communicating with the second pressure changing chamber 13 from an ambient air side is formed, the bimetal 11 reduces the volume of the first pressure changing chamber 12 during a high temperature and increases the same during a low temperature.