PURPOSE: To make a turning support unit structure of a steering knuckle compact, and also improve lubricability and durability.

CONSTITUTION: In a driving wheel steering device where a steering knuckle 29 is supported with an axle housing 34 so as to be capable of turning, an upward extending axle end 37 is fixed to the end part of the axle housing 34, and pins 38 and 39 are also implanted in the end part of the axle end 37. An upward extending support arm 36 is provided above the steering knuckle 29, and in an above position of a constant velocity joint 33 to transmit driving force, the support arm 36 is supported with the pins 38 and 39 so as to be capable of turning through bearings 40 and 41, and there are also provided seals 42 and 43 so as to surround a support unit by means of the bearing 40 and 41.