Provided is a water filtering device for a humidifier capable of increasing the filling amount of an ion exchange resin by including a separate inner cylinder having upper and lower diameters that are different from each other, increasing adsorption and removal efficiency of mineral components contained in water by taking a long ionic bonding time between the water and the ion exchange resin, and effectively preventing scales of the mineral components from being extracted by heating a heater to prevent malfunction of the heater or pipes.