To provide a steam trap capable of easily washing foreign matters adhered and deposited on a screen.

A body 3 is formed with an inlet 1, a valve chamber 9 and an outlet 2. A ball vale element 5 of a hollow spherical shape is kept rotatable between the inlet 1 and the valve chamber 9. A filter part 4 is installed inside the ball valve element 5. At positions almost orthogonal to the filter part 4 of the ball valve element 5, a valve chamber communication path 6 and an inlet communication path 7 are formed respectively. A blow passage 8 is formed in a lower end part of the body 3. By rotating the ball valve element 5 counterclockwise by a handle not shown or the like so that the inlet communication path 7 communicates with the inlet 1 and a lower end inner peripheral part of the filter part 4 communicates with the blow passage 8, foreign matters accumulated on the inner peripheral face of the filter part 4 can be back-washed.