A humidity control system comprising: a refrigerant circuit including a compressor, a first heat exchanger carrying an adsorbent thereon, an expansion valve adjustable in opening and a second heat exchanger carrying an adsorbent thereon, the humidity control system being configured to supply humidity-controlled air to a room by operating alternately in a first batch mode in which the second heat exchanger adsorbs moisture in air and the first heat exchanger releases moisture to air and a second batch mode in which the first heat exchanger adsorbs moisture in air and the second heat exchanger releases moisture to air, and an opening control means for controlling the opening of the expansion valve; characterized in that the humidity control system comprises: an initialization means configured to set the initial opening control value of the opening control means for each of the first and second batch modes by setting, upon start of the first batch mode, the initial opening control value for the expansion valve to the opening of the expansion valve at the end of the preceding first batch mode and setting, upon start of the second batch mode, the initial opening control value for the expansion valve to the opening of the expansion valve at the end of the preceding second batch mode, wherein the opening control means is configured to execute, after the start of each of the batch modes, a control keeping the opening of the expansion valve at a fixed degree until a predetermined time immediately before the end of each batch mode has passed, and the opening control means is configured to execute a control, after the predetermined period has passed in the control, to change the opening of the expansion valve so that the degree of superheat of the refrigerant in the refrigerant circuit reaches the specified valve. a refrigerant circuit including a compressor, a first heat exchanger carrying an adsorbent thereon, an expansion valve adjustable in opening and a second heat exchanger carrying an adsorbent thereon, the humidity control system being configured to supply humidity-controlled air to a room by operating alternately in a first batch mode in which the second heat exchanger adsorbs moisture in air and the first heat exchanger releases moisture to air and a second batch mode in which the first heat exchanger adsorbs moisture in air and the second heat exchanger releases moisture to air, and an opening control means for controlling the opening of the expansion valve; characterized in that the humidity control system comprises: an initialization means configured to set the initial opening control value of the opening control means for each of the first and second batch modes by setting, upon start of the first batch mode, the initial opening control value for the expansion valve to the opening of the expansion valve at the end of the preceding first batch mode and setting, upon start of the second batch mode, the initial opening control value for the expansion valve to the opening of the expansion valve at the end of the preceding second batch mode, wherein the opening control means is configured to execute, after the start of each of the batch modes, a control keeping the opening of the expansion valve at a fixed degree until a predetermined time immediately before the end of each batch mode has passed, and the opening control means is configured to execute a control, after the predetermined period has passed in the control, to change the opening of the expansion valve so that the degree of superheat of the refrigerant in the refrigerant circuit reaches the specified valve. an initialization means configured to set the initial opening control value of the opening control means for each of the first and second batch modes by setting, upon start of the first batch mode, the initial opening control value for the expansion valve to the opening of the expansion valve at the end of the preceding first batch mode and setting, upon start of the second batch mode, the initial opening control value for the expansion valve to the opening of the expansion valve at the end of the preceding second batch mode, wherein the opening control means is configured to execute, after the start of each of the batch modes, a control keeping the opening of the expansion valve at a fixed degree until a predetermined time immediately before the end of each batch mode has passed, and the opening control means is configured to execute a control, after the predetermined period has passed in the control, to change the opening of the expansion valve so that the degree of superheat of the refrigerant in the refrigerant circuit reaches the specified valve.