A DC electric power system (1A, 1B, 1C, 1D), including: a load apparatus that consumes or converts DC electric power that is input to an input terminal; an electric path connected to the input terminal; a DC electric power source string in which a plurality of DC electric power source units are connected, the DC electric power source units each having a DC electric power source module that generates electric power or performs charging/discharging and an optimizer that has two output terminals (22a, 22c), is connected in series to the DC electric power source module and optimizes an output voltage of the DC electric power source module and supplies the optimized output voltage to the electric path; and an arc detection apparatus, comprising: for each of the plurality of optimizers, a first capacitor whose one end portion is connected to one output terminal of the DC electric power source module, and whose other end portion is connected to one output terminal of the respective one of the plurality of optimizers; an electric current measurement unit that measures an electric current on the electric path between the DC electric power source string and the load apparatus; and an arc determination unit that determines whether or not an arc is generated, based on a high frequency component of an AC current measured by the electric current measurement unit; wherein the optimizers are connected via their respective two output terminals (22a, 22c) in series to each other and in series to the electric path, such that the DC electric power source units are connected in series to each other; and the optimizer is provided for each series connection of a plurality of solar battery cells that constitute one DC electric power source module. a load apparatus that consumes or converts DC electric power that is input to an input terminal; an electric path connected to the input terminal; a DC electric power source string in which a plurality of DC electric power source units are connected, the DC electric power source units each having a DC electric power source module that generates electric power or performs charging/discharging and an optimizer that has two output terminals (22a, 22c), is connected in series to the DC electric power source module and optimizes an output voltage of the DC electric power source module and supplies the optimized output voltage to the electric path; and an arc detection apparatus, comprising: for each of the plurality of optimizers, a first capacitor whose one end portion is connected to one output terminal of the DC electric power source module, and whose other end portion is connected to one output terminal of the respective one of the plurality of optimizers; an electric current measurement unit that measures an electric current on the electric path between the DC electric power source string and the load apparatus; and an arc determination unit that determines whether or not an arc is generated, based on a high frequency component of an AC current measured by the electric current measurement unit; wherein the optimizers are connected via their respective two output terminals (22a, 22c) in series to each other and in series to the electric path, such that the DC electric power source units are connected in series to each other; and the optimizer is provided for each series connection of a plurality of solar battery cells that constitute one DC electric power source module. for each of the plurality of optimizers, a first capacitor whose one end portion is connected to one output terminal of the DC electric power source module, and whose other end portion is connected to one output terminal of the respective one of the plurality of optimizers; an electric current measurement unit that measures an electric current on the electric path between the DC electric power source string and the load apparatus; and an arc determination unit that determines whether or not an arc is generated, based on a high frequency component of an AC current measured by the electric current measurement unit; wherein the optimizers are connected via their respective two output terminals (22a, 22c) in series to each other and in series to the electric path, such that the DC electric power source units are connected in series to each other; and the optimizer is provided for each series connection of a plurality of solar battery cells that constitute one DC electric power source module.