A nuclear power plant operation support device, comprising: a virtual plant creation unit arranged for acquiring operating state data on an actual plant, simulating the actual plant and creating a virtual plant, the virtual plant creation unit reading the operating state data with a given period and reflecting the operating state data upon the virtual plant such that an operating state of the actual plant is conformed to that of the virtual plant; a reference data storage unit for storing in advance reference data for creating the virtual plant conforming to various operating states of the actual plant; an operating state monitoring unit arranged for monitoring the operating state of the actual plant and sending the operating state data on the actual plant to the virtual plant creation unit, while acquiring from the reference data storage unit the reference data corresponding to the operating state, if occurs, in the operating state of the actual plant and sending the reference data to the virtual plant creation unit; an operating state prediction unit arranged for creating a predicted virtual plant in which an operating state after a lapse of arbitrary time is predicted by accelerating the operating state of the virtual plant created by the virtual plant creation unit at an arbitrary speed, the operating state after the lapse of arbitrary time being predicted by the operating state prediction unit; and an operating state displaying unit for displaying the operating state of the actual plant and the operating state of the predicted virtual plant on an operating state display unit, characterized by further comprising: an operating state diagnosis unit arranged for diagnosing the operating state of the actual plant, while diagnosing the operating state of the predicted virtual plant; and a diagnosis result displaying unit arranged for displaying a diagnosis result of the actual plant and a diagnosis result of the predicted virtual plant on an diagnosis result display unit. a virtual plant creation unit arranged for acquiring operating state data on an actual plant, simulating the actual plant and creating a virtual plant, the virtual plant creation unit reading the operating state data with a given period and reflecting the operating state data upon the virtual plant such that an operating state of the actual plant is conformed to that of the virtual plant; a reference data storage unit for storing in advance reference data for creating the virtual plant conforming to various operating states of the actual plant; an operating state monitoring unit arranged for monitoring the operating state of the actual plant and sending the operating state data on the actual plant to the virtual plant creation unit, while acquiring from the reference data storage unit the reference data corresponding to the operating state, if occurs, in the operating state of the actual plant and sending the reference data to the virtual plant creation unit; an operating state prediction unit arranged for creating a predicted virtual plant in which an operating state after a lapse of arbitrary time is predicted by accelerating the operating state of the virtual plant created by the virtual plant creation unit at an arbitrary speed, the operating state after the lapse of arbitrary time being predicted by the operating state prediction unit; and an operating state displaying unit for displaying the operating state of the actual plant and the operating state of the predicted virtual plant on an operating state display unit, an operating state diagnosis unit arranged for diagnosing the operating state of the actual plant, while diagnosing the operating state of the predicted virtual plant; and a diagnosis result displaying unit arranged for displaying a diagnosis result of the actual plant and a diagnosis result of the predicted virtual plant on an diagnosis result display unit.