To provide a flat type drain trap which can be used without causing a delay in the closure of a valve over a wide pressure range from low to high pressure, and which can increase a draining capacity at high pressure.

A casing having a valve chest 3 therein is formed by a body 1 having an inlet 4, an outlet passage 5 and an outlet 6, and a lid 2 fastened to the body 1. A first valve seat 8 with a first valve port 7 open to the lower sidewall of the valve chest 3 is formed. A second valve seat 10 having a second valve port 9 smaller in diameter than the first valve port 7 and reducing the area of passage through the first valve port 7 while in a position displaced to the valve chest 3 is positioned on the outlet 6 side of the first valve port 7 in such a way as to be capable of moving back and forth. An inlet chamber 12 communicating with the back of the second valve seat 10 from the valve chest 3 is formed and a spring 13 causing the second valve seat 10 to exert a valve opening force for increasing the area of passage through the first valve port 7 is positioned therein. A float 15 which opens and closes the first valve port 7 while the second valve seat 10 increases the area of passage through the first valve port 7 from the outlet 6 and which opens and closes the second valve port 9 smaller in diameter than the first valve port 7 while the second valve seat 10 reduces the area of passage through the first valve port 7 from the outlet 6 is positioned within the valve chest 3.