An electric power steering apparatus comprising: a motor for generating auxiliary torque corresponding to steering torque exerted by a driver of a vehicle; and a control unit for controlling the motor, wherein the motor and the control unit are integrally coupled with each other and the motor is mounted in the vehicle through the intermediary of a gear unit, wherein the motor has a motor housing in which motor constituent members are contained, one axis-direction end portion (7a1) of which is sealed, and the other axis-direction end portion (7a2) of which is opened, wherein the control unit has a control unit case in which control apparatus constituent members are contained, one axis-direction end portion (6a1) of which is opened, and the other axis-direction end portion (6a2) of which is sealed, wherein the other axis-direction end portion (7a2) of the motor housing and the one axis-direction end portion (6a1) of the control unit case are coupled with each other while maintaining a waterproofing function, wherein the one axis-direction end portion (7a1) of the motor housing and the gear unit are coupled with each other while maintaining a waterproofing function, wherein an output axle of the motor extends to the outside of the motor housing while maintaining a waterproofing function, and wherein the motor housing has a respiratory apparatus that performs a respiratory action, based on a pressure change in at least one of the motor housing and the control unit wherein the motor housing is provided with a bearing that has a waterproofing function and pivotably supports the output axle, characterized in that wherein the motor housing is coupled with the gear unit through the intermediary of an O-ring, and wherein the respiratory apparatus is disposed at a position that is between the O-ring and the bearing and is upper in the vertical direction than the output axle. a motor for generating auxiliary torque corresponding to steering torque exerted by a driver of a vehicle; and a control unit for controlling the motor, wherein the motor and the control unit are integrally coupled with each other and the motor is mounted in the vehicle through the intermediary of a gear unit, wherein the motor has a motor housing in which motor constituent members are contained, one axis-direction end portion (7a1) of which is sealed, and the other axis-direction end portion (7a2) of which is opened, wherein the control unit has a control unit case in which control apparatus constituent members are contained, one axis-direction end portion (6a1) of which is opened, and the other axis-direction end portion (6a2) of which is sealed, wherein the other axis-direction end portion (7a2) of the motor housing and the one axis-direction end portion (6a1) of the control unit case are coupled with each other while maintaining a waterproofing function, wherein the one axis-direction end portion (7a1) of the motor housing and the gear unit are coupled with each other while maintaining a waterproofing function, wherein an output axle of the motor extends to the outside of the motor housing while maintaining a waterproofing function, and wherein the motor housing has a respiratory apparatus that performs a respiratory action, based on a pressure change in at least one of the motor housing and the control unit wherein the motor housing is provided with a bearing that has a waterproofing function and pivotably supports the output axle, characterized in that wherein the motor housing is coupled with the gear unit through the intermediary of an O-ring, and wherein the respiratory apparatus is disposed at a position that is between the O-ring and the bearing and is upper in the vertical direction than the output axle.