An oil cooling device for a server tower, comprising: the server tower having a flow passage through which insulating cooling oil flows inside the tower; a heat absorbing part for cooling a heat generating electronic device within the server tower; a cooling oil tube outside the tower connected from the flow passage of the server tower and passing through a hot water generating water tank, an air heater and a residual-heat processing part outside the tower; an insulating cooling oil in the cooling oil tube and flow passage for preventing the electronic devices within the server tower from being damaged even though the oil leaks; a hydraulic pump outside the tower for circulating the cooling oil in the cooling oil tube; a first temperature sensor disposed in the server tower adapted to detect an air temperature within the tower; the heater allowing the cooling oil tube passed through the hot water generating water tank to contact air in a position at which heating is required, thereby performing a secondary cooling process; and a second temperature sensor disposed in the cooling oil tube in which the second cooling process is performed to measure a temperature of the cooling oil tube; wherein, the oil cooling device further comprises: the hot water generating water tank allowing the cooling oil tube to contact cooling water in the hot water generating water tank, thereby performing a primary cooling process; the residual-heat processing part having a small-scaled and low-power cooler disposed in the cooling oil tube in which the second cooling process is performed to perform a tertiary cooling process; and a control unit for controlling operations of the hydraulic pump and the residual-heat processing part according to a value measured from each of the first and second temperature sensors (120, 310) on the basis of predefined reference temperature data; wherein, the hot water generating tank comprises a warm air supply fan on an outer surface thereof to supply warm air in the tank into an indoor space through a duct connected to the tank. the server tower having a flow passage through which insulating cooling oil flows inside the tower; a heat absorbing part for cooling a heat generating electronic device within the server tower; a cooling oil tube outside the tower connected from the flow passage of the server tower and passing through a hot water generating water tank, an air heater and a residual-heat processing part outside the tower; an insulating cooling oil in the cooling oil tube and flow passage for preventing the electronic devices within the server tower from being damaged even though the oil leaks; a hydraulic pump outside the tower for circulating the cooling oil in the cooling oil tube; a first temperature sensor disposed in the server tower adapted to detect an air temperature within the tower; the heater allowing the cooling oil tube passed through the hot water generating water tank to contact air in a position at which heating is required, thereby performing a secondary cooling process; and a second temperature sensor disposed in the cooling oil tube in which the second cooling process is performed to measure a temperature of the cooling oil tube; wherein, the oil cooling device further comprises: the hot water generating water tank allowing the cooling oil tube to contact cooling water in the hot water generating water tank, thereby performing a primary cooling process; the residual-heat processing part having a small-scaled and low-power cooler disposed in the cooling oil tube in which the second cooling process is performed to perform a tertiary cooling process; and a control unit for controlling operations of the hydraulic pump and the residual-heat processing part according to a value measured from each of the first and second temperature sensors (120, 310) on the basis of predefined reference temperature data; the hot water generating water tank allowing the cooling oil tube to contact cooling water in the hot water generating water tank, thereby performing a primary cooling process; the residual-heat processing part having a small-scaled and low-power cooler disposed in the cooling oil tube in which the second cooling process is performed to perform a tertiary cooling process; and a control unit for controlling operations of the hydraulic pump and the residual-heat processing part according to a value measured from each of the first and second temperature sensors (120, 310) on the basis of predefined reference temperature data; wherein, the hot water generating tank comprises a warm air supply fan on an outer surface thereof to supply warm air in the tank into an indoor space through a duct connected to the tank.