Abstract: Provided is a desiccant dehumidifier characterized in that: external air is introduced through a cooling coil and an eliminator; the introduced external air is bifurcated into a desiccant rotor and a bypass line in front of the desiccant rotor; the external air bifurcated to the desiccant rotor passes through the desiccant rotor disposed at a dehumidifying section to perform dehumidification, and the bypass air passed through the bypass line is mixed with the air passed through the desiccant rotor behind the desiccant rotor to form suction air to be supplied through a suction fan; and a part of the external air bifurcated to the desiccant rotor passes through the desiccant rotor disposed at a purge section, and then passes through the desiccant rotor disposed at a recycling section via a recycling heater to form exhaust air to be exhausted through an exhaust fan, wherein ventilation air is introduced through a separate path on a path through which the external air is introduced and then mixed with the externa