To achieve energy saving in the cooling towers as a whole in a heat source system including a plurality of cooling towers connected in parallel to a common chiller. In a heat source system, a plurality of cooling towers are connected in parallel to a common chiller. The cooling towers cool cooling water used and thereby heated in the chiller, and supplies the cooled cooling water to the chiller. A cooling tower control device for controlling the cooling towers starts all the cooling towers at the start of the heat source system.