An initial abnormality analysis data is reliably retained by a controller of an electric power steering apparatus. The controller of the electric power steering apparatus is constituted by comprising abnormality detection device for detecting the abnormality of a steering assist control mechanism comprising an electric motor giving a steering assist force to a steering system and abnormality data storage device for storing abnormality analysis data in storage device when the abnormality of the steering assist control mechanism is detected by this abnormality detection device, wherein the storage device comprises a plurality of overwrite prohibition storage areas for prohibiting rig the overwrite of the abnormality analysis data and a plurality of overwrite allowable storage areas for overwriting and storing the abnormality analysis data, and wherein the abnormality data storage device stores the abnormality of the steering assist control mechanism detected by the abnormality detection device in the overwrite prohibition storage area when the abnormality is a first abnormality analysis data, and stores the abnormality in the overwrite allowable storage area when the abnormality is the abnormality analysis data subsequent to the second time.