An in wheel motor driving device which has a deceased axial dimension and includes aluminum alloy housings having a minimum wall thickness for reduced weight while retaining good assemblability. An in-wheel motor driving device includes: a motor section which has an output shaft; a wheel hub bearing section C; and a speed reducer section which connects the output shaft of the motor section and the wheel hub bearing section coaxially with each other. A housing outer circumferential portion of the motor section and a housing outer circumferential portion of the speed reducer section are integral with each other, being provided by one, outer circumferential member. An outboard-side end member is fixed to an outboard-side end of the outer circumferential member by means of bolts, and the wheel hub bearing section has its fixing ring fixed to the outboard-side end member.