A humidity control device for an air-conditioning system including an indoor unit that has an indoor heat exchanger and an indoor unit expansion valve, the humidity control device comprising: a compressor; a condenser; an expansion device; first and second water adsorption/desorption devices (33a, 33b) that are disposed in an air path providing communication between an air inlet through which air flows in from a humidity controlled space and an air outlet through which air flows out into the humidity controlled space, the first and second water adsorption/desorption devices (33a, 33b) each including an adsorbent whose equilibrium adsorption capacity with respect to air having a relative humidity in a range of 40% through 100% increases substantially linearly with an increase in the relative humidity, the first and second water adsorption/desorption devices (33a, 33b) being configured to release water into air having a relatively low humidity and adsorb water from air having a relatively high humidity; an evaporator disposed between the first water adsorption/desorption device and the second water adsorption/desorption device in the air path and connected in parallel with the indoor heat exchanger; and a switching device that switches between a channel in which the air that has flowed in through the air inlet is caused to pass through the first water adsorption/desorption device, the evaporator, and the second water adsorption/desorption device in this order, and a channel in which the air that has flowed in through the air inlet is caused to pass through the second water adsorption/desorption device, the evaporator, and the first water adsorption/desorption device in this order. a compressor; a condenser; an expansion device; first and second water adsorption/desorption devices (33a, 33b) that are disposed in an air path providing communication between an air inlet through which air flows in from a humidity controlled space and an air outlet through which air flows out into the humidity controlled space, the first and second water adsorption/desorption devices (33a, 33b) each including an adsorbent whose equilibrium adsorption capacity with respect to air having a relative humidity in a range of 40% through 100% increases substantially linearly with an increase in the relative humidity, the first and second water adsorption/desorption devices (33a, 33b) being configured to release water into air having a relatively low humidity and adsorb water from air having a relatively high humidity; an evaporator disposed between the first water adsorption/desorption device and the second water adsorption/desorption device in the air path and connected in parallel with the indoor heat exchanger; and a switching device that switches between a channel in which the air that has flowed in through the air inlet is caused to pass through the first water adsorption/desorption device, the evaporator, and the second water adsorption/desorption device in this order, and a channel in which the air that has flowed in through the air inlet is caused to pass through the second water adsorption/desorption device, the evaporator, and the first water adsorption/desorption device in this order.