A device for processing an input data of a protective relay, the device comprising: a voltage and current detecting part configured to detect a voltage and current on an electric line; an analog-digital converting part configured to sample an analog signal inputted through the voltage and current detecting part by a predetermined sampling frequency and then convert the sampled analog input signal to digital data; a memory part configured to store event data, fault data, wave data, and demand data, according to a predetermined control, which are inputted respectively through the voltage and current detecting part; characterized by a digital signal processing part including a measurement accumulation buffer and an update accumulation buffer, wherein the digital signal processing part is configured to: count and accumulate the number of samples of the digital data from the analog-digital converting part, compare the counted and accumulated number of samples with a value of one clock cycle; responsive to determining that the counted and accumulated number of samples is less than the value of one clock cycle, execute an Recursive Discrete Fourier Transform, RDFT, operation on the digital data from the analog-digital converting part and store a result of the RDFT operation in the measurement accumulation buffer and the update accumulation buffer; and responsive to determining that the counted and accumulated number of samples is equal to the value of one clock cycle, replace the result of the RDFT operation stored in the measurement accumulation buffer with the result of the RDFT operation stored in the update accumulation buffer, and initialize the result of the RDFT operation stored in the update accumulation buffer and the counted and accumulated number of samples. a voltage and current detecting part configured to detect a voltage and current on an electric line; an analog-digital converting part configured to sample an analog signal inputted through the voltage and current detecting part by a predetermined sampling frequency and then convert the sampled analog input signal to digital data; a memory part configured to store event data, fault data, wave data, and demand data, according to a predetermined control, which are inputted respectively through the voltage and current detecting part; characterized by a digital signal processing part including a measurement accumulation buffer and an update accumulation buffer, wherein the digital signal processing part is configured to: count and accumulate the number of samples of the digital data from the analog-digital converting part, compare the counted and accumulated number of samples with a value of one clock cycle; responsive to determining that the counted and accumulated number of samples is less than the value of one clock cycle, execute an Recursive Discrete Fourier Transform, RDFT, operation on the digital data from the analog-digital converting part and store a result of the RDFT operation in the measurement accumulation buffer and the update accumulation buffer; and responsive to determining that the counted and accumulated number of samples is equal to the value of one clock cycle, replace the result of the RDFT operation stored in the measurement accumulation buffer with the result of the RDFT operation stored in the update accumulation buffer, and initialize the result of the RDFT operation stored in the update accumulation buffer and the counted and accumulated number of samples. count and accumulate the number of samples of the digital data from the analog-digital converting part, compare the counted and accumulated number of samples with a value of one clock cycle; responsive to determining that the counted and accumulated number of samples is less than the value of one clock cycle, execute an Recursive Discrete Fourier Transform, RDFT, operation on the digital data from the analog-digital converting part and store a result of the RDFT operation in the measurement accumulation buffer and the update accumulation buffer; and responsive to determining that the counted and accumulated number of samples is equal to the value of one clock cycle, replace the result of the RDFT operation stored in the measurement accumulation buffer with the result of the RDFT operation stored in the update accumulation buffer, and initialize the result of the RDFT operation stored in the update accumulation buffer and the counted and accumulated number of samples.