An in-wheel motor driving device in which a motor section A and a wheel hub bearing section C are connected coaxially in series via a speed reducer section B, whereby an axial dimension of the in-wheel motor driving device is reduced to provide a large inside space of a vehicle as well as an increase in freedom of routing a power supply wires. A terminal box for the power supply wires which supply power to the motor section A is disposed on the outer circumferential side surface of the housing which holds the motor section A. The power supply wires are routed out of the terminal box to an inboard side and are anchored by a power supply wire holder which extends perpendicularly to an axial centerline of the housing.