To provide a disc type steam trap capable of increasing pressure in an annular groove and outlet passage in a short time when a valve is opened.

A lid member 5 is mounted in a body 3 having an inlet 1 and an outlet 2 through the medium of a valve seat member 4 to form a pressure changing chamber 6 between the valve seat member 4 and lid member 5. A valve disc 9 is arranged in the pressure changing chamber 6. The valve seat member 4 comprises an inlet passage 15 opened in a center of the pressure changing chamber 6 and communicated from the inlet 1 side, an annular groove 16 opened in the outer periphery of the inlet passage 15, an inner ring valve seat 7 formed between the inlet passage 15 and annular groove 16, an outer ring valve seat 8 formed in the outer periphery of the annular groove 16, an outlet passage 17 opening one end to the annular groove 16, and a communication hole 19 having a fluid passing area smaller than a fluid passing area of the outlet passage 17 and connecting the other end of the outlet passage 17 to the outlet 2 to communicate.