To provide a steam trap capable of preventing erosion of an outlet passage.

In a steam trap having a valve chamber 6 having an inlet 3 opened at an upper part and the outlet passage 5 separated from the valve chamber 6 and having an outlet 4 opened at an upper part formed on casings 1, 2, having a valve seat member 12 having a valve hole 10 opened on an end in the valve chamber 6 side and a branch hole 11 opened on an end in the outlet passage 5 side provided in an lower part of the outlet passage 5 to keep communication between the valve chamber 6 and the outlet passage 5, and having a spherical float 19 opening and closing the valve hole 10 arranged in the valve chamber 6, vent holes 15a, 15b, 15c and 15d facing each other are opened on an roughly cylindrical circumference wall 14, return water flow from the valve hole 10 flows in the vent holes 15a, 15b, 15c and 15d from an outside, a vessel made of erosion resistant material to which return water flow flowing inward flows out from the branch hole 10 is arranged in the valve hole 10, return water reservoir part 8 having a mesh member 21 arranged therein is connected to a lower part of the outlet passage 5 and the branch hole 11 is opened toward the return water reservoir part 8.