A control method for a hydraulically driven fan using a control device of a hydraulically driven fan which is provided with a hydraulic motor, a hydraulically driven fan driven by the hydraulic motor and a control valve for varying a flow rate of hydraulic oil to be supplied to the hydraulic motor according to an applied valve controlled variable, in which a fan speed command (Nf) is feedback-controlled according to a control deviation (ε) between a target fan speed (Nt) of the hydraulically driven fan and a feedback quantity, wherein the method comprises; a) calculating the target fan speed (Nt) based on an engine water temperature (tw), a hydraulic oil temperature (to) and a number of engine rotations (ne), detecting an actual fan speed (Nd), and calculating a difference between the target fan speed (Nt) and the actual fan speed (Nd) as a control deviation (ε), characterised in that the method further comprises: b) controlling the control valve by calculating a target valve controlled variable (I n) based on the target fan speed (Nt) when the actual fan speed (Nd) is zero, and calculating the valve controlled variable to gradually reach toward the calculated target valve controlled variable (I n) with a prescribed variation (Δi) per prescribed unit time, c) calculating the fan speed command (Nf) by using a proportional action based on the control deviation (ε) while a difference (ΔI) between the target valve controlled variable (I n) and an actual valve controlled variable (I n-1) is not smaller than a predetermined threshold, d) calculating the fan speed command (Nf) by using the proportional action and an integral action based on the control deviation (ε) while the difference (ΔI) between the target valve controlled variable (I n) and the actual valve controlled variable (I n-1) is smaller than the predetermined threshold, and e) controlling the control valve by calculating the target valve controlled variable (I n) based on the calculated fan speed command (Nf) and calculating the valve controlled variable to gradually reach toward the calculated target valve controlled variable (I n) with a prescribed variation (Δi) per prescribed unit time. a) calculating the target fan speed (Nt) based on an engine water temperature (tw), a hydraulic oil temperature (to) and a number of engine rotations (ne), detecting an actual fan speed (Nd), and calculating a difference between the target fan speed (Nt) and the actual fan speed (Nd) as a control deviation (ε), characterised in that the method further comprises: b) controlling the control valve by calculating a target valve controlled variable (I n) based on the target fan speed (Nt) when the actual fan speed (Nd) is zero, and calculating the valve controlled variable to gradually reach toward the calculated target valve controlled variable (I n) with a prescribed variation (Δi) per prescribed unit time, c) calculating the fan speed command (Nf) by using a proportional action based on the control deviation (ε) while a difference (ΔI) between the target valve controlled variable (I n) and an actual valve controlled variable (I n-1) is not smaller than a predetermined threshold, d) calculating the fan speed command (Nf) by using the proportional action and an integral action based on the control deviation (ε) while the difference (ΔI) between the target valve controlled variable (I n) and the actual valve controlled variable (I n-1) is smaller than the predetermined threshold, and e) controlling the control valve by calculating the target valve controlled variable (I n) based on the calculated fan speed command (Nf) and calculating the valve controlled variable to gradually reach toward the calculated target valve controlled variable (I n) with a prescribed variation (Δi) per prescribed unit time.