A nuclear reactor fuel integrity monitor comprising: a γ-ray detector configured to detect γ-ray of a specific radionuclide of a subject measurement medium of a nuclear reactor; a sample container configured to retain the subject measurement medium therein and surrounding the circumference of the γ-ray detector; and a measurement control device configured to perform a control so that a predetermined amount of the subject measurement medium is introduced into the sample container, and configured to calculate a concentration of the specific radionuclide from γ-ray data per each unit time detected by the γ-ray detector and a volume of the subject measurement medium introduced into the sample container, wherein the sample container includes a recess portion into which the γ-ray detector is inserted such that the subject measurement medium retained in said sample container surrounds the circumference of the γ-ray detector, wherein the γ-ray detector is a Ge-semiconductor detector, wherein the nuclear reactor fuel integrity monitor further comprises a cooling device configured to cool the γ-ray detector to a temperature as low as that of the liquid nitrogen and connected to the Ge-semiconductor detector so as to reduce a current leaking to the Ge-semiconductor detector or a noise, characterized in that the subject measurement medium is a coolant of the nuclear reactor and the specific radionuclide is radioactive iodine, and in that an inlet side of the sample container is provided with an activated alumina column, and the coolant passing through the activated alumina column is introduced into the sample container. a γ-ray detector configured to detect γ-ray of a specific radionuclide of a subject measurement medium of a nuclear reactor; a sample container configured to retain the subject measurement medium therein and surrounding the circumference of the γ-ray detector; and a measurement control device configured to perform a control so that a predetermined amount of the subject measurement medium is introduced into the sample container, and configured to calculate a concentration of the specific radionuclide from γ-ray data per each unit time detected by the γ-ray detector and a volume of the subject measurement medium introduced into the sample container, wherein the sample container includes a recess portion into which the γ-ray detector is inserted such that the subject measurement medium retained in said sample container surrounds the circumference of the γ-ray detector, wherein the γ-ray detector is a Ge-semiconductor detector, wherein the nuclear reactor fuel integrity monitor further comprises a cooling device configured to cool the γ-ray detector to a temperature as low as that of the liquid nitrogen and connected to the Ge-semiconductor detector so as to reduce a current leaking to the Ge-semiconductor detector or a noise, characterized in that the subject measurement medium is a coolant of the nuclear reactor and the specific radionuclide is radioactive iodine, and in that an inlet side of the sample container is provided with an activated alumina column, and the coolant passing through the activated alumina column is introduced into the sample container.