An air-cooled condenser comprising a distributing chamber (23) for distributing a vaporous medium to be condensed, a condensate collecting chamber (24) and finned tubes (17) with fins on air side, said finned tubes (17) being connected in parallel between the distributing chamber (23) and the condensate collecting chamber (24), where each of the finned tubes (17) comprises two parallel essentially flat side walls and exterior closings connecting the side walls, in the finned tubes (17) there are longitudinal separation walls (18) connected to the side walls and dividing the inner space of the finned tubes (17) into longitudinal parallel channels (19), and in the separation walls (18) there are breakthroughs (27) for allowing the flow of the medium between neighbouring channels (19).