A method and a device for attaching and removing an additional device having individual elements to and from a main boom of a mobile crane using no auxiliary crane is provided. In the method, a partial piece of the additional device having partial pieces is raised into an inclined position while being rotated about an upper pivot point, and a head of the main boom is bolted to a lower end of the inclined end of the partial piece of the additional device facing the mobile crane so as to be able to rotate. Using a hoist cable, all partial pieces are raised in alignment with respect to each other and the main boom, and are bolted to each other in this position and to the main boom. A displacement device for carrying out the method is also provided.