The invention relates to a structure of a rotation device, a rotation device, and a forest machine. The rotation device includes - a first inner body part (12) comprising first conduits (26) fitted to the first inner body part (12) for leading a pressure medium through the first inner body part (12), - a second outer body part (14), arranged at least partly around the first inner body part (12) completely rotatably, comprising connections (28) fitted to the second outer body part (14), - second conduits (30) arranged to lead the pressure medium from the said first conduit (26) to the said connections (28), - an axial conduit (16) for leading electrical conductors (56) through the first inner body part (12) and comprising two ends (16.1, 16.2), - bearing means (20) arranged to permit the second outer body part (14) to rotate relative to the first inner body part (12) and vice versa, and - suspension means (90) for suspending the structure (10) of the rotation device (1) from a boom (2). The structure (10) according to the invention includes attachment means (22) for attaching a direct-drive pressure-medium operated turning device (24) to the structure (10) coaxially in order to transmit torque. In the structure (10) there is an axial conduit (16) extending from the suspension means (90) through the structure (10) and the turning device (24) attached to it for carrying electric conductors (56).
