PURPOSE: To increase catching rate of vapor in a vapor trap of a liquid metal fast breeder by providing a rim to a catching mesh for preventing bypassing of vapor.

CONSTITUTION: The gas contg. liquid metal vapor is passed through a mesh unit 9' section for catching vapor constructed by piling in multilayer in a trap vessel 1 from a inlet nozzle 3, to catch the liquid metal vapor on the strand wires of the mesh, and the gas is discharged from a nozzle outlet 3. The rim 9'b of the mesh unit 9' is supported tightly by an annular stopper 14 protruded to the inside wall of the vessel 1 to prevent bypassing of the gas through the gap of the mesh unit 9'.