A reactor state monitoring apparatus (1; 31) comprising: a device (17, 23) for measuring radiation outside a shielding that is adapted to detect and to measure a radiation dose in an atmosphere caused by radioactive substances which are on a containment vessel side with respect to a biological shielding wall surrounding a pressure vessel and which are outside the pressure vessel; a device (15, 21) for measuring radiation inside the shielding that is adapted to detect and to measure a radiation dose on the pressure vessel side with respect to the biological shielding wall; a dosage/water level diagram storage device that is adapted to store a dosage/water level diagram indicating a relation between measurement values of the device (15, 21) for measuring radiation inside the shielding and water levels corresponding to water amounts of a cooling water in the pressure vessel obtained in advance; a fuel radiation measuring device (16, 21) that is adapted to detect and to measure a radiation dose of a fuel contained in the pressure vessel; and a water amount evaluation device that is adapted to calculate a difference between the radiation dose measured by the fuel radiation measuring device (16, 21) and the radiation dose measured by the device (17, 23) for measuring radiation outside the shielding as a corrected fuel radiation dose, the water amount evaluation device further being adapted to read out the dosage/water level diagram from the dosage/water level diagram storage device, to correct the read dosage/water level diagram with use of the corrected fuel radiation dose, and to acquire a water level of the cooling water corresponding to the radiation dose measured by the device (15, 21) for measuring radiation inside the shielding based on the corrected dosage/water level diagram as a water level evaluation value. a device (17, 23) for measuring radiation outside a shielding that is adapted to detect and to measure a radiation dose in an atmosphere caused by radioactive substances which are on a containment vessel side with respect to a biological shielding wall surrounding a pressure vessel and which are outside the pressure vessel; a device (15, 21) for measuring radiation inside the shielding that is adapted to detect and to measure a radiation dose on the pressure vessel side with respect to the biological shielding wall; a dosage/water level diagram storage device that is adapted to store a dosage/water level diagram indicating a relation between measurement values of the device (15, 21) for measuring radiation inside the shielding and water levels corresponding to water amounts of a cooling water in the pressure vessel obtained in advance; a fuel radiation measuring device (16, 21) that is adapted to detect and to measure a radiation dose of a fuel contained in the pressure vessel; and a water amount evaluation device that is adapted to calculate a difference between the radiation dose measured by the fuel radiation measuring device (16, 21) and the radiation dose measured by the device (17, 23) for measuring radiation outside the shielding as a corrected fuel radiation dose, the water amount evaluation device further being adapted to read out the dosage/water level diagram from the dosage/water level diagram storage device, to correct the read dosage/water level diagram with use of the corrected fuel radiation dose, and to acquire a water level of the cooling water corresponding to the radiation dose measured by the device (15, 21) for measuring radiation inside the shielding based on the corrected dosage/water level diagram as a water level evaluation value.