A plant state displaying apparatus adapted for displaying information representing a state of a plant, the plant state displaying apparatus comprising: a generator configured to acquire plant information from at least one of devices, apparatuses, and facilities installed in the plant, calculate a state value representing the state of the plant using the plant information, and generate a plant state diagram in which at least one of a color and a density of the at least one of devices, apparatuses, and facilities from which the plant information is acquired is changed according to magnitude of the state value; a display configured to display the plant state diagram generated by the generator as the information representing the state of the plant; a setter configured to set a display condition for displaying the plant state diagram on the display, the display condition defining magnitude of a degree of degradation of the at least one of said devices, apparatuses, and facilities represented with the at least one of a color and a density changed; and a storage; wherein the generator is configured to generate the plant state diagram in which information representing operation ranges of a plurality of workers operating in the plant is superimposed on a diagram representing the plant, the generator is configured to store the generated plant state diagram in the storage together with time information representing generation time, read the plant state diagram stored in the storage based on instructions from outside of the plant state displaying apparatus, and display the read plant state diagram on the display based on the time information, the generator being further configured to perform animation display of the plant state diagrams read from the storage on the display in order of a time series, wherein the plant state diagrams read from the storage include a plant state diagram at a first time when operation of the plant is started or when restarting of the plant is started and a plant state diagram at a second time that is after the first time. a generator configured to acquire plant information from at least one of devices, apparatuses, and facilities installed in the plant, calculate a state value representing the state of the plant using the plant information, and generate a plant state diagram in which at least one of a color and a density of the at least one of devices, apparatuses, and facilities from which the plant information is acquired is changed according to magnitude of the state value; a display configured to display the plant state diagram generated by the generator as the information representing the state of the plant; a setter configured to set a display condition for displaying the plant state diagram on the display, the display condition defining magnitude of a degree of degradation of the at least one of said devices, apparatuses, and facilities represented with the at least one of a color and a density changed; and a storage; wherein the generator is configured to generate the plant state diagram in which information representing operation ranges of a plurality of workers operating in the plant is superimposed on a diagram representing the plant, the generator is configured to store the generated plant state diagram in the storage together with time information representing generation time, read the plant state diagram stored in the storage based on instructions from outside of the plant state displaying apparatus, and display the read plant state diagram on the display based on the time information, the generator being further configured to perform animation display of the plant state diagrams read from the storage on the display in order of a time series, wherein the plant state diagrams read from the storage include a plant state diagram at a first time when operation of the plant is started or when restarting of the plant is started and a plant state diagram at a second time that is after the first time.