A merging unit configured to collect information of at least current or voltage from a power system, comprising: a processing unit configured to process an analog signal obtained from said power system and to output digital data indicating a state of the power system; and a communication unit configured to communicate with an external device (5, 8) via a communication line, wherein said processing unit includes a first conversion unit configured to convert an analog signal of the current or the voltage of said power system to digital data with a first sample rate, and a second conversion unit configured to convert the digital data with said first sample rate received from said first conversion unit to digital data with a second sample rate lower than said first sample rate, wherein the first sample rate is not an integer multiple of the second sample rate, wherein the second conversion unit comprises; an up-conversion unit configured to convert the digital data with said first sample rate received from said first conversion unit to digital data with a sample rate of a least common multiple of said first sample rate and a second sample rate, and a down-conversion unit configured to receive the digital data converted by said up-conversion unit and to convert the received digital data to the digital data with said second sample rate, wherein said up-conversion unit has an interpolation unit configured to interpolate the digital data with said first sample rate from said first conversion unit, by N1 times, where N1 = the sample rate of said least common multiple divided by said first sample rate, by inserting, between a sample value and a next sample value, data identical to one of the sample values, and said processing unit is configured to store the digital data with said first sample rate converted by said first conversion unit, over a prescribed period which is based on timing at which said merging unit receives a prescribed signal via said communication unit, and to transmit the digital data with said second sample rate converted by said second conversion unit, via said communication unit. a processing unit configured to process an analog signal obtained from said power system and to output digital data indicating a state of the power system; and a communication unit configured to communicate with an external device (5, 8) via a communication line, wherein said processing unit includes a first conversion unit configured to convert an analog signal of the current or the voltage of said power system to digital data with a first sample rate, and a second conversion unit configured to convert the digital data with said first sample rate received from said first conversion unit to digital data with a second sample rate lower than said first sample rate, wherein the first sample rate is not an integer multiple of the second sample rate, wherein the second conversion unit comprises; an up-conversion unit configured to convert the digital data with said first sample rate received from said first conversion unit to digital data with a sample rate of a least common multiple of said first sample rate and a second sample rate, and a down-conversion unit configured to receive the digital data converted by said up-conversion unit and to convert the received digital data to the digital data with said second sample rate, wherein said up-conversion unit has an interpolation unit configured to interpolate the digital data with said first sample rate from said first conversion unit, by N1 times, where N1 = the sample rate of said least common multiple divided by said first sample rate, by inserting, between a sample value and a next sample value, data identical to one of the sample values, and said processing unit is configured to store the digital data with said first sample rate converted by said first conversion unit, over a prescribed period which is based on timing at which said merging unit receives a prescribed signal via said communication unit, and to transmit the digital data with said second sample rate converted by said second conversion unit, via said communication unit.