The surface tension of an aqueous solution of piperidolate hydrochloride (PIH), which is an anti-cholinergic drug, was measured with a Du Nouy tensiometer at 25°C in the absence and presence of acetylcholine chloride(Ach·Cl), and the effect of Ach·Cl on the critical micelle concentration (cmc) of PIH was investigated by comparing it with the effects of sodium chloride (NaCl) and calcium chloride (CaCl₂). Furthermore, the effect of Ach·Cl on the cmc of PIH was discussed by comparing it with the effect of Ach·Cl on the cmc of dodecyltrimethylammonium chloride (DTAC). The value of cmc of PIH decreased with an increasing concentration of Ach·Cl. The order of the effect on the decrease in the cmc of PIH was CaCl₂>NaCl>Ach·Cl. The effect of Ach·Cl on the decrease in the cmc of PIH was smaller than that on the decrease in the cmc of DTAC.