PURPOSE: To allow a heat transfer area to freely conform to steam turbine operating conditions by providing a tertiary element to be connected at one end to an intermediate header in an air-cooled condenser in which steam cooled by a primary element is introduced to a secondary element through the header and again cooled.

CONSTITUTION: An air-cooled condenser is used for generating or other steam turbine plant, steam air-cooled by a primary element 1 is introduced to a secondary element 2 through an intermediate header 6, and again air-cooled. In this case, a tertiary element is provided at intermediate between the adjacent elements 1 and 2, and an upper end side of the element 3 is connected to an exhaust tube 5 of a steam turbine and an outlet header 2a of the element 2 through switching units 7a, 7b. A downstream side of the element 3 is connected to the header 6. Thus, the units 7a, 7b are switched to perform the element 3 as a function of part of the elements 1, 2.