The purpose of the present invention is to provide an efficient production method for dyed fibers that reduces undyed regions when dying raw material fiber containing a silicon component and to provide an agent for preventing undyed regions, that can be suitably used in said method. The production method for dyed fiber includes a step for dying the raw material fiber that contains a silicon component, in a dye bath that includes a dye and at least one kind of ester selected from a phosphoric acid ester (A) indicated by general formula (1) and a sulfate ester (B) indicated by general formula (2).