To provide a braking force adjusting mechanism for a turning auxiliary brake in mobile agricultural machinery, capable of adjusting rear wheel braking force by a hydraulic actuator with simple mechanical constitution.

Mobile agricultural machinery has front wheels and rear wheels; front wheel accelerating mechanism for accelerating the front wheels when a steering angle becomes a specified angle or more during turning; and brake mechanism for making a brake selectively act upon the rear wheel on the turning inner side. The brake mechanism has braking devices respectively provided on the lateral rear wheels. The respective braking devices have link mechanism 33, 33a, 33c and hydraulic actuators 22R, 22L for driving the link mechanism. When the steering angle becomes the specified angle or more, the hydraulic actuators 22R, 22L are driven to drive the link mechanism 33, 33a, 33c, and braking force selectively acts upon the rear wheels through the braking devices. A coil spring 31 is provided between the actuators 22R, 22L and the link mechanism 33, 33a, 33c, and the link mechanism is driven through the coil spring 31.