To provide a disc type steam trap which can effectively insulate a transforming chamber without wasting steam on an inlet side.

In this disc type steam trap in which a transforming chamber 8 is formed in the inside by inner/outer ring valve seats 12, 13 and an inner cover 7, an injection hole 9 is opened in the center of the inner ring valve seat 12 so as to communicate an inlet 2 and the transforming chamber 8 with each other, an annular groove 10 is arranged between the inner ring valve seat 12 and the outer ring valve seat 13, a discharge hole 11 is opened from the annular groove 10 so as to communicate the transforming chamber 8 with an outlet 3, a valve disc 14 separated/seated from/on the inner/outer ring valve seats 12, 13 is arranged in the inside of the transforming chamber 8 and a heat insulating chamber 17 is formed on the outer side of the transforming chamber 8, an outlet communicating passage 16 communicating the heat insulating chamber 17 with the outlet 3 side is arranged, and the heat insulating chamber 17 side opening of the outlet communicating passage 16 is formed in the upper part of the heat insulating chamber 17.