According to one embodiment of a reactor core monitoring system, includes: an information retention portion for retaining a regular cycle and a short cycle as calculation information of reactor core performance data; a signal processing portion for creating heat balance data based on a process signal; a data acquisition portion for acquiring, in a timing of the regular cycle, the heat balance data and reactor core performance data which was calculated in a previous timing of the regular cycle, while acquiring, in a timing of the short cycle asynchronous to the regular cycle, the heat balance data and reactor core performance data which was calculated most recently; and a data calculation portion for calculating new reactor core performance data based on the acquired reactor core performance data and the heat balance data.