Co-phase traction power supply system was proposed for supplying the long-distance electrified railway without neutral sections. Active power conditioner (APC) is required in each substation, so that the traction loads are supplied without deteriorates the power quality at the grid side. The back-to-back converter was used to implement the APC in the past studies. However, bulky and costly coupling transformers have to be used. In this paper, the APC is implemented by using a three-phase modular multilevel converter (MMC). The MMC allows a simple cascading of sub-modules and the output voltage level can be easily increased. As a result, no coupling transformer is required. The proposed APC using MMC compensates reactive power, harmonics and unbalance currents of the traction loads. Simulation results are provided to show the validity of the proposed system and its control.