PROBLEM TO BE SOLVED: To obtain a fiber insect pest controller capable of protecting fibers such as wool, silk, cotton and flax from a fiber insect pest, hardly exerting a bad influence on environment by using a hydrazine derivative. SOLUTION: This fiber insect pest controller comprises at least one selected from hydrazine derivatives of formula I [A and B are O, S or a group of formula II (R is H, an alkyl, etc.; R' is H, an alkyl, etc.), a group of formula III, C(=O), a group of formula IV or NR'; R to R are each H, a halogen, an alkyl, an alkoxyalkyl, etc.; R to R are each H, a halogen, an alkyl, nitro, amino, etc.; R to R are each H, a halogen, an alkyl a trialkylsilyloxyalkyl, nitro, etc.; R is H, cyano, a haloalkylthio, an acyl, etc.; R is an alkyl; (l), (m) and (n) are each 0 or 1]. The content of the active ingredient of the controller is about 0.1-50wt.%. More excellent insecticidal effect can be developed by blending two or more kinds of the derivatives as compounds to be used here. The controller can be mixed with another physiologically active substance and used.