A method for integrated online monitoring for a nuclear power plant for enabling the transmission of signals of various types of measuring instruments installed in machines of a plurality of units of said nuclear power plant to a centralized integrated monitoring center located at a remote location using an integrated online monitoring system including a monitoring data relay device, a diagnostic data relay device and a local data acquisition device, the method comprising: a first step of collecting machine monitoring data of said plurality of units in the nuclear power plant selectively from the local data acquisition device via the monitoring data relay device; and a second step of collecting machine diagnostic data of said plurality of units in the nuclear power plant selectively from the local data acquisition device via the diagnostic data relay device; wherein the first step includes: transmitting the machine monitoring data to the monitoring data relay device from a power plant monitoring system via an unidirectional communication cable, wherein the unidirectional communication cable has a cable output to exterior and a cable input to the nuclear power plant therein, the cable input to the nuclear power plant therein is configured so that input communication cannot be performed physically; and synchronizing a file format and a communication scheme of the machine monitoring data received in the monitoring data relay device and transmitting the machine monitoring data to the local data acquisition device. a first step of collecting machine monitoring data of said plurality of units in the nuclear power plant selectively from the local data acquisition device via the monitoring data relay device; and a second step of collecting machine diagnostic data of said plurality of units in the nuclear power plant selectively from the local data acquisition device via the diagnostic data relay device; wherein the first step includes: transmitting the machine monitoring data to the monitoring data relay device from a power plant monitoring system via an unidirectional communication cable, wherein the unidirectional communication cable has a cable output to exterior and a cable input to the nuclear power plant therein, the cable input to the nuclear power plant therein is configured so that input communication cannot be performed physically; and synchronizing a file format and a communication scheme of the machine monitoring data received in the monitoring data relay device and transmitting the machine monitoring data to the local data acquisition device. transmitting the machine monitoring data to the monitoring data relay device from a power plant monitoring system via an unidirectional communication cable, wherein the unidirectional communication cable has a cable output to exterior and a cable input to the nuclear power plant therein, the cable input to the nuclear power plant therein is configured so that input communication cannot be performed physically; and synchronizing a file format and a communication scheme of the machine monitoring data received in the monitoring data relay device and transmitting the machine monitoring data to the local data acquisition device.