To obtain a pest-controlling agent containing a specific quarternary ammonium salt type compound, providing a specific range of pH when dissolved in water, excellent in washability when the agent is held by a substrate such as a fiber, and hardly providing a bed effect to instruments used when the agent is held.

This pest-controlling agent contains a quarternary ammonium salt type compound of the formula [R1 to R4 are each an alkyl or an alkenyl, and one or two of R1 to R4 are each an 8-20 group and the others are each a 1-3C group; Xn- is an anion or the like of malic acid, succinic acid, citric acid, maleic acid or the like; (n) and (m) are each 1-4], and provides an aqueous solution with pH 6.0-8.5. The substrate is preferably fiber, cotton, rush, paper, wood, wadding, a straw mat or a TATAMI mat (mat made of rush for Japanese life).