According to an embodiment, an oscillation power range monitor system has a plural of OPRM units. Each of the OPRM units has: the receiving cell-output set signals and averaging the cell-output set signals; a time average calculating unit calculating a time average cell value; a normalized cell value calculating unit calculating a normalized cell value; a trip determining unit outputting a reactor trip signal if amplitude or growth rate of oscillation or period of oscillation of the normalized cell value has exceeded a prescribed condition; a signal and prescribed value adjusting unit adjusting the relation between the normalized cell value and the specific value, thereby compensating for the deterioration in the neutron flux sensitivity of any LPRM detector in order to keep outputting the reactor trip signal.