A thermosiphon which can be manufactured easily at low costs, having excellent resistance to pressure, without the circulation of a working fluid being hindered. A condenser 3 includes a condensing section 4 composed of extruded members in which a plurality of fine pores 7 are formed, a branching section 5 provided on an upstream side of the fine pores 7 to supply a gaseous working fluid returned from a gas pipe 12 into each of the fine pores 7, and a collecting section 6 provided on a downstream side of the fine pores 7 to collect the working fluid condensed inside the fine pores 7 and then supply the same into a liquid pipe 9. The gas pipe 12 is connected to an upper portion of the branching section 5 and the liquid pipe 9 is connected to a lower portion of the collecting section 6.