To provide a feed water heater in which superheated steam flows, that is, the feed water heater capable of effectively operating a heating pipe for heating feed water.

This feed water heater has a body 1 and the heating pipe 20 arranged in the body 1. The superheated steam is introduced into a spray chamber 10 arranged upstream of a steam introducing port of the body 1, and a condensate of the feed water heater is sprayed on the superheated steam by a spray nozzle 11, and is formed as saturated steam by removing an overheating degree. The saturated steam introduced into the body 1 condenses by contacting with the heating pipe 20, and heats the feed water in the heating pipe 20 by latent heat discharged by condensation. Since the steam introduced into the body 1 contacts with the heating pipe as the saturated steam by removing the overheating degree, a heat transfer coefficient is secured large in the whole area. Power generation output is also controlled by controlling a spray quantity of the condensate.