A safety device for a machine having an action part for processing or moving an object includes: a posture obtainment unit configured to obtain a plurality of posture data indicating a posture of at least part of the lower body of a worker; and a preparatory movement determination unit configured to determine that a preparatory movement preceding a dangerous movement which brings at least part of the upper body of the worker closer to a working area of the action part is being performed in the case where a change in the posture indicated by the plurality of posture data shows that the worker is shifting a center of gravity of whole body of the worker in an orientation away from the working area of the action part without changing positions of both feet.