A capacity control system for a variable capacity compressor includes target suction pressure setting means for setting, based on external information input from external information detection means, a target suction pressure for controlling the pressure in a suction pressure region as a control target, suction pressure estimation means for estimating, based on the external information, a pressure that is estimated to prevail in the suction pressure region if the variable capacity compressor is operated with a maximum discharge capacity, and discharge capacity determination means for determining, based on the suction pressure estimated by the suction pressure estimation means and the target suction pressure set by the target suction pressure setting means, whether the variable capacity compressor is operating with the maximum discharge capacity or with a controlled discharge capacity.