In an intelligent electronic device, a first receiving circuit receives first time-series data at a first sample rate representing the electrical quantity of an electric power system from a merging unit. An up converter converts the first time-series data into second time-series data having a second sample rate higher than the first sample rate by interpolating the first time-series data. A down converter converts the second time-series data into third time-series data having a third sample rate lower than the first sample rate by periodically extracting a data point at any changeable sample time from the second time-series data. A relay computer performs protective relay computation using third time-series data.