To provide an in-wheel motor system equipped with a waterproofing mechanism to prevent the water from intruding into an in-wheel motor of hollow shape while the constitution remains simple.

The in-wheel motor system is structured so that the space in the in-wheel motor 3 having a hollow shape and the space in a vehicle body 50 are coupled together through a hollow pipe 31, a pressure regulating valve 32 and an air pump 33 are coupled with the body side end 31m of the pipe 31, and that a raindrop sensor 34 is installed under the motor 3, wherein when the sensor 34 has sensed the motor 3 being covered with water, the air pump 33 is put in operation to make pressurization of the pressure in the motor 3 to a specified pressure as high as not permitting water intrusion so that the waterproofness of the motor 3 is secured easily.