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

A pressure changing chamber is formed of a body 1 and a lid member 10 in the inside, an entrance 2 and the pressure changing chamber are communicated with each other by arranging 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 concentrical 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 the exhit 3 by forming the 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 setting on the valve seat 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 the first pressure changing chamber 12 where the valve disk 17 is arranged and the second pressure changing chamber in 13 as a hot 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 the exit 3 side is formed, the bimetal 11 reduces the volume of the first pressure changing chamber 12 at a high temperature and increases the same at a low temperature.