A human hair fiber treatment agent for treating human hair fibers separated from the human head and artificially fixed at one part of the longitudinal direction, wherein the human hair fiber treatment agent comprises the following components (A) to (C) in the formulation thereof, and the content of the component (A) is 1% by mass or more: (A): formaldehyde or a hydrate thereof; (B): a melamine derivative represented by the formula (1) wherein R.sup.1 to R.sup.3 each represent a hydrogen atom, a hydroxymethylamino group, a hydroxy group, a halogen atom, a phenyl group, a linear or branched alkyl group or alkenyl group having 1 or more and 6 or less carbon atoms, or a linear or branched alkoxy group or alkenyloxy group having 1 or more and 6 or less carbon atoms; and (C): water.

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A cutting and double-face leather integrated dyeing process for Tan sheep skin, which comprises the following steps: cutting.fwdarw.washing with water.fwdarw.retanning.fwdarw.primary dyeing.fwdarw.primary color fixing.fwdarw.secondary dyeing.fwdarw.fatliquoring.fwdarw.secondary color fixing.fwdarw.bleaching.fwdarw.drying.fwdarw.a finished product. Tan sheep skin leather boards and fleeces dyed by the process of the invention have no cross color and color fading with each other, a high color fastness, and uniform dyeing; the prepared Tan sheep skin products such as clothing, etc., not only have the characteristics of a high added value and good practicability, but also can more largely increase product benefits and lower the production cost.