The objective of the present invention is to achieve conservation of energy for all of the cooling towers in a heat source system wherein multiple cooling towers are connected in parallel to a common refrigerating machine. In a heat source system (1) multiple cooling towers (3a, 3b, and 3c) are connected in parallel to a common refrigerating machine (2). The cooling towers (3a, 3b, and 3c) cool cooling water that has been heated by being used in the refrigerating machine (2), and supply the cooled water to the refrigerating machine (2). A cooling tower control device (4) that controls the cooling towers (3a, 3b, and 3c) activates all of the cooling towers (3a, 3b, and 3c) when the heat source system (1) is activated.