Provided is a drive unit for a windmill, wherein, if a plurality of drive units are used, even when an excessive external force is applied to one drive unit, the drive unit can be prevented from being destroyed; and if a recessed portion functioning as a mechanism for preventing the drive unit from being destroyed is provided, the recessed portion can be easily replaced. A recessed portion (19) is provided in a drive force transmission path, i.e., a path for transmitting a drive force from an output shaft (14) to a pinion (13), and functions as a disconnection mechanism for disconnecting the connection to the drive force transmission path when a torque is applied to the recessed portion at a predetermined or greater level. A section (20) is provided on the inside of a main body (11), and is provided between the recessed portion (19) and a speed reduction unit (17), to define an area between the recessed portion (19) and the speed reduction unit (17).