To provide a cold heat source system allowing reduction of cooling water pump power when a necessary cooling load is small or when cooling water has the minimum flow rate.

In this cold heat source system 70, the cooling water sent from equipment 54 through a going path pipe 32 by a cooling water pump 56 is cooled in a cross-flow cooling tower 50, and is returned to the equipment 54 through a return path pipe 56. The cooling tower 50 has: a plurality of heat exchange units 48a, 48b vertically disposed inside a cooling tower body 20 in a multistage, cooling the cooling water by sprinkling the cooling water sent from the equipment 54 over water sprinkle tanks each filled with a filler 26a (26b) from an upper water tank 6, bring it into contact with outside air introduced into the cooling tower body 20 to perform heat exchange, and allowing water conduction of the cooling water from a topmost stage to a bottommost stage of multiple stages; a changeover means changing over a water sending destination of the cooling water from the equipment 54 to the plurality of heat exchange units 48a, 48b; and a control means 58 controlling the changeover of the changeover means according to the cooling load of the cooling water cooled in the cooling tower.