To provide a disk steam trap capable of accommodating wide-ranging steam pressures as far as possible.

An inlet 1 and an outlet 3 are provided in a body 2, a blowoff port 6 is opened in a variable pressure chamber 5 formed in the body, an inner ring valve seat 7 is formed on an outer circumferential edge of the blowoff port, an annular groove 8 is provided on the outer circumference of the inner ring valve seat, an outer ring valve seat 9 on a same plane with the inner ring valve seat is formed on the outer circumference edge of the annular groove, an exhaust port 10 is opened at a bottom part of the annular groove, a disk valve 11 is disposed in the variable pressure chamber, and drain W is exhausted to the exhaust port 10 as the disk valve is detached/attached from/to the inner ring valve seat and the outer ring valve seat due to change of steam pressure in the variable pressure chamber. An expansion chamber 12 is formed at a lower portion of the exhaust port, a drain exhausted from the exhaust port stays in the expansion chamber, flash steam is generated from the drain in the expansion chamber, and steam pressure Q increased by the flash steam reaches the pressure chamber 5.