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 . <EOS>