An in-wheel motor drive mechanism comprising: a hub configured to be mounted to a wheel body of a vehicle wheel; an electric motor disposed inwardly of the hub in a width direction of a vehicle including the vehicle wheel so as to be coaxial with the hub; a motor housing in which is mounted the electric motor; a hub bearing mounted to an outer end of the motor housing in the width direction of the vehicle and rotatably supporting the hub; a brake disk mounted to the hub so as to rotate together with the hub; a pair of brake pads disposed on respective sides of the brake disk; and a brake caliper configured to clamp the brake disk by means of the brake pads, characterized in that the brake caliper is fixed to an outer periphery of the motor housing by driving threaded members radially inwardly of the motor housing from radially outside of the motor housing, wherein the motor housing includes a large-diameter portion having an outer diameter larger than the diameter of the outer periphery of the brake disk, an end plate located at the outer end of the large-diameter portion 13A in the width direction of the vehicle, of the motor housing; wherein the brake caliper includes: a caliper bracket 18A including a pair of base pieces fixed to the outer periphery of the large-diameter portion of the motor housing; a pair of inwardly protruding pieces extending radially inwardly along the end plate of the motor housing from outer ends, in the width direction of the vehicle, of the respective base pieces; and a coupling arm through which the pair of inwardly protruding pieces are coupled together; and wherein the inwardly protruding pieces are in abutment with the end plate of the motor housing. a hub configured to be mounted to a wheel body of a vehicle wheel; an electric motor disposed inwardly of the hub in a width direction of a vehicle including the vehicle wheel so as to be coaxial with the hub; a motor housing in which is mounted the electric motor; a hub bearing mounted to an outer end of the motor housing in the width direction of the vehicle and rotatably supporting the hub; a brake disk mounted to the hub so as to rotate together with the hub; a pair of brake pads disposed on respective sides of the brake disk; and a brake caliper configured to clamp the brake disk by means of the brake pads, characterized in that the brake caliper is fixed to an outer periphery of the motor housing by driving threaded members radially inwardly of the motor housing from radially outside of the motor housing, wherein the motor housing includes a large-diameter portion having an outer diameter larger than the diameter of the outer periphery of the brake disk, an end plate located at the outer end of the large-diameter portion 13A in the width direction of the vehicle, of the motor housing; wherein the brake caliper includes: a caliper bracket 18A including a pair of base pieces fixed to the outer periphery of the large-diameter portion of the motor housing; a pair of inwardly protruding pieces extending radially inwardly along the end plate of the motor housing from outer ends, in the width direction of the vehicle, of the respective base pieces; and a coupling arm through which the pair of inwardly protruding pieces are coupled together; and wherein the inwardly protruding pieces are in abutment with the end plate of the motor housing.