To provide a swing device and a hydraulic circuit of a clamshell bucket capable of horizontally swinging a bucket suspended from the tip of an expansible boom.

This clamshell bucket capable of grasping sediment by a pair of laterally rotating buckets 43 and 44 comprises a swinging mechanism 35 having a suspended upper part and a lower part rotatable in circumferential direction, a mast 36 connected to the lower part of the swinging mechanism 35, the pair of buckets 43 and 44 pivotally supported on the lower part of the mast 36 so as to be rotated in lateral direction, and a pair of hydraulic cylinders installed between the upper part of the mast 36 and the buckets 43 and 44 to rotate the buckets 43 and 44. The swinging mechanism 35 comprises a suspended cylindrical fixed body 68 positioned at the upper part thereof and a cylindrical driven body 69 positioned at the lower part thereof and forming an opening. The fixed body 68 is inserted into the opening of the driven body 69 to fix it so as to be rotated in circumferential direction. A hydraulic motor 102 for rotating the fixed body 68 and the driven body 69 each other is installed between the fixed body and the driven body.