An in-wheel motor drive device includes: a motor part; a speed reduction part; a wheel bearing part; and a casing, the motor part including: a stator, which is fixed to the casing; a rotation shaft of a motor, which is rotatably supported on the casing through intermediation of a plurality of rolling bearings; and a rotor, which is mounted to the rotation shaft of the motor, the rotation shaft of the motor being configured to rotationally drive an input shaft of a speed reducer, the speed reduction part being configured to reduce a speed of rotation of the input shaft of the speed reducer to transmit the rotation to an output shaft of the speed reducer, the wheel bearing part being connected to the output shaft of the speed reducer, the rotation shaft of the motor being made of case hardening steel and subjected to carburizing, quenching, and tempering.