To provide a treating method for making a pitch fiber infusible, capable of markedly reducing a time required for the treatment under an oxidizing atmosphere by adding quinones to a carbonaceous pitch and melt spinning.

This treating method for making a pitch fiber infusible, is to add 0.01-50wt.% quinones such as pyrenequinone and phenanthrene to a carbonaceous pitch capable of melt spinning, and melt spin it. It is possible to improve a productivity by reducing the time required for the following treatment for making the fiber infusible without adding any specific process.