A plant abnormality indication monitoring system for detecting an abnormality indication in a nuclear power plant that performs a base-load operation so that an instrument parameter has a predefined target value, comprising: a data storage unit configured to store plant operation data including an instrument parameter measured in the nuclear power plant and an alarm threshold for a warning of abnormality in the nuclear power plant; and a monitoring control unit configured to detect an abnormality indication in the nuclear power plant on the basis of the plant operation data, wherein the monitoring control unit is further configured to set a region having values equal to or smaller than the alarm threshold as a normality determination range in which the instrument parameter is normal and a region having values greater than the alarm threshold as an abnormality determination range in which the instrument parameter is abnormal, to set, in the normality determination range, a normal range which is a range for the instrument parameter in a current predetermined period of the nuclear power plant based on past plant operation data and to to store the normal range in the data storage unit in advance, and is configured to detect an abnormality indication in a case where the instrument parameter at the current time exceeds the set normal range, wherein ranges μ ± 3σ and μ ± 5σ are set to be included in the normality determination range as normal ranges and μ + 3σ and μ + 5σ, which are upper values of the respective normal ranges, are equal to or less than the alarm threshold K, wherein σ is a standard deviation of the instrument parameter and μ is an average value of the instrument parameter in a past predetermined period of the nuclear power plant, characterized in that the monitoring control unit is further configured to change the normal range from μ ± 5σ to μ ± 3σ to be reduced from the early period to the late period of the current predetermined period of the nuclear power plant. a data storage unit configured to store plant operation data including an instrument parameter measured in the nuclear power plant and an alarm threshold for a warning of abnormality in the nuclear power plant; and a monitoring control unit configured to detect an abnormality indication in the nuclear power plant on the basis of the plant operation data, wherein the monitoring control unit is further configured to set a region having values equal to or smaller than the alarm threshold as a normality determination range in which the instrument parameter is normal and a region having values greater than the alarm threshold as an abnormality determination range in which the instrument parameter is abnormal, to set, in the normality determination range, a normal range which is a range for the instrument parameter in a current predetermined period of the nuclear power plant based on past plant operation data and to to store the normal range in the data storage unit in advance, and is configured to detect an abnormality indication in a case where the instrument parameter at the current time exceeds the set normal range, wherein ranges μ ± 3σ and μ ± 5σ are set to be included in the normality determination range as normal ranges and μ + 3σ and μ + 5σ, which are upper values of the respective normal ranges, are equal to or less than the alarm threshold K, wherein σ is a standard deviation of the instrument parameter and μ is an average value of the instrument parameter in a past predetermined period of the nuclear power plant, characterized in that the monitoring control unit is further configured to change the normal range from μ ± 5σ to μ ± 3σ to be reduced from the early period to the late period of the current predetermined period of the nuclear power plant.