PURPOSE: To enhance workability and safety by arranging installing brackets on a support plate and a rotary plate of a work machine tool installing device, fixing pivotally a center pin to the support plate and the rotary plate so as to be mountable and demountable, arranging a driving means and a transmission device on the support plate and the rotary plate, and arranging a torque transmission restricting unit in the transmission device.

CONSTITUTION: A center pin 22 is pivotally fixed to a support plate 1 and a rotary plate 2 so as to be mountable and demountable. An arm installing pin 6 and a link installing pin 5 are inserted in and fixed to a pair of brackets 3 of the support plate 1. The center pin 22 is pivotally fixed to the support plate 1 and the rotary plate 2 so as to be mountable and demountable, and the rotary plate 2 is turned by using the center pin 22 as a shaft. A motor 19 is arranged on the support plate 1, and a work machine tool such as a bucket is turned and actuated. Transmission of driving force of the motor 19 to an annular gear 31 from a gear 30 is restricted. When torque applied to the work machine tool 4 is removed, a steel ball 48 pressed by a spring 47 engages with either recessed part 49 by being pressed by the spring 47. Thereby, the driving force of the motor 19 is transmitted to the annular gear 31 from the gear 30, and rotates the rotary plate 2.