The claimed invention relates to a filtering device for purifying a liquid, intended for softening and purifying tap water and other liquids for domestic use. A device for purifying a liquid, having an inlet channel and an outlet channel, consists of at least two tanks, one for a softening agent and one for a regenerating solution, and a control mechanism including a housing, which consists of at least three parts which form chambers and liquid-conducting cavities, at least two pistons, which are located in the chambers and which are linked by means of a cam mechanism to an engine, a saline valve, and a discharge valve; the control mechanism is designed such that the pistons can move simultaneously in the chambers in opposite directions, at the same time alternately shutting off the liquid-conducting cavities in the housing; at least two liquid-conducting cavities are formed between the indicated portions of the housing, and at least one cavity is formed between the closest portion of the housing to the upper portion of the tank for the softening agent and the upper portion of the tank for the softening agent. The technical result consists in increasing the reliability and simplifying the design of a device for purifying a liquid, and in decreasing the weight of the device for purifying a liquid.