Abstract: According to the invention, a fluorination treated substance remarkably reduced in the light absorption loss of a substance to be treated, such as a fluoride thin film, a fluorination treatment apparatus capable of producing the fluorination treated substance, and a process for producing a fluorination treated substance can be provided. Disclosed is a process for producing a fluorination treated substance, comprising an enclosure step of enclosing a substance to be treated in a reactor and a fluorination reaction step of introducing a fluorine gas into the reactor to bring the substance to be treated into contact with the fluorine gas and thereby carry out fluorination reaction.