In an excavator equipped with an offset boom, an offset command value generating device of a rotation control device offsets a second boom arranged on a distal end side in a rotation direction relative to a first boom arranged on a proximal end side when an acceleration start judging device judges that a rotation operation is started, and offsets the second boom in a reverse rotation direction when a deceleration start judging device judges that a rotation deceleration operation is started. Accordingly, when a rotation acceleration is performed using a reaction force generated in the offset, clearances between members of a work machine can be contracted in advance in the rotation direction, while when the rotation deceleration operation is performed, these clearances can be contracted in advance in the rotation reverse direction, thereby reducing an impact in the acceleration and deceleration.