A device for detecting an open phase in a connection line of a standby transformer in a nuclear power plant, the device comprising: a Rogowski coil installed around a neutral line of a primary Y connection line in the standby transformer used in the nuclear power plant; an open phase detector configured to determine that the open phase is generated in a primary side of the standby transformer by determining whether an intensity of the current induced in the Rogowski coil maintains a preset reference value or greater for a preset reference time or greater while a secondary circuit breaker of the standby transformer is open, and configured to generate an alarm signal in response to open phase information; and a main controller configured to display an alarm to a manager according to an alarm signal provided from the open phase detector, wherein the open phase detector comprises: a current detector configured to measure an intensity of the current induced in the Rogowski coil, and configured to close a contact point-A when the intensity of the current is equal to or greater than the reference value, and configured to open the contact point-A when the intensity of the current is less than the reference value; a cut-off detector configured to close a contact point-B when the secondary circuit breaker of the standby transformer is open, and configured to open the contact point-B when the secondary circuit breaker of the standby transformer is closed; and a timer configured to start an operation when the contact point-A and the contact point-B are closed and to close a contact point-C when the preset reference time comes; and an alarm signal generator configured to close a contact point-D corresponding to the alarm signal when the contact-C is closed, wherein the main controller is configured to display the alarm when the contact point-D is closed. a Rogowski coil installed around a neutral line of a primary Y connection line in the standby transformer used in the nuclear power plant; an open phase detector configured to determine that the open phase is generated in a primary side of the standby transformer by determining whether an intensity of the current induced in the Rogowski coil maintains a preset reference value or greater for a preset reference time or greater while a secondary circuit breaker of the standby transformer is open, and configured to generate an alarm signal in response to open phase information; and a main controller configured to display an alarm to a manager according to an alarm signal provided from the open phase detector, wherein the open phase detector comprises: a current detector configured to measure an intensity of the current induced in the Rogowski coil, and configured to close a contact point-A when the intensity of the current is equal to or greater than the reference value, and configured to open the contact point-A when the intensity of the current is less than the reference value; a cut-off detector configured to close a contact point-B when the secondary circuit breaker of the standby transformer is open, and configured to open the contact point-B when the secondary circuit breaker of the standby transformer is closed; and a timer configured to start an operation when the contact point-A and the contact point-B are closed and to close a contact point-C when the preset reference time comes; and an alarm signal generator configured to close a contact point-D corresponding to the alarm signal when the contact-C is closed, wherein the main controller is configured to display the alarm when the contact point-D is closed. a current detector configured to measure an intensity of the current induced in the Rogowski coil, and configured to close a contact point-A when the intensity of the current is equal to or greater than the reference value, and configured to open the contact point-A when the intensity of the current is less than the reference value; a cut-off detector configured to close a contact point-B when the secondary circuit breaker of the standby transformer is open, and configured to open the contact point-B when the secondary circuit breaker of the standby transformer is closed; and a timer configured to start an operation when the contact point-A and the contact point-B are closed and to close a contact point-C when the preset reference time comes; and an alarm signal generator configured to close a contact point-D corresponding to the alarm signal when the contact-C is closed, wherein the main controller is configured to display the alarm when the contact point-D is closed.