To provide the subject apparatus capable of efficiently heat-treating food material(s) through controlling in high accuracy the wetness as well as heating temperatures of the steam to be fed.

This apparatus is designed in such a scheme that a steam feed pipe 9 is connected through a control valve 2 to a heater 1, and a gas feed pipe 25 and a liquid feed pipe 26 are connected to the steam feed pipe 9; both the temperature and wetness of the steam to be fed are controlled by feeding a gas such as air or a liquid such as hot water to the steam in the steam feed pipe 9.