Disclosed is a control device of a hydraulically driven fan. The control device of a hydraulically driven fan is characterized by comprising first command value calculation means for calculating a fan speed command by a proportional term only while a difference between a target valve controlled variable and an actual valve controlled variable is not smaller than a predetermined threshold, second command value calculation means for calculating a fan speed command by the proportional term and an integral term while a difference between a target valve controlled variable and an actual valve controlled variable is smaller than a predetermined threshold, target valve controlled variable calculation means for calculating a target valve controlled variable of a control valve based on the calculated fan speed command, and control means for outputting a valve controlled variable to a control valve such that the control valve reaches the target valve controlled variable.