To provide a free float type drain pipe allowing a protrusion to be readily fit into a groove provided in a trap casing.

An inlet 5, a trap chamber 6 and an outlet 7 are formed by a trap casing comprised of a body 1 and a lid member 2. A drain valve hole 16 connecting the trap chamber 6 with the outlet 7 is provided below a trap chamber. In the trap chamber 6, a float 19 for opening/closing the drain valve hole 16 is arranged. A cylindrical screen 9 is arranged above the trap chamber 6 and a float cover 12 in an inversed bowl shape having a through hole 10 provided and deviated to the inlet 5 side is arranged inside the cylindrical screen 9. The screen 9 and the float cover 12 are fixed and a protrusion 13 with its lower end bent inside is fixed to a lower end of the screen 9. By fitting the protrusion 13 into a groove 18 provided in the body 1, rotation of the float cover 12 is prevented.