A balancer mechanism for a rotating shaft is configured to prevent deviation of the axis of the rotating shaft. The balancer mechanism (20) has an assistance torque generation device (21) for generating an assistance torque. The assistance torque generation device (21) includes a cam member (24) fixed to the rotating shaft (4), a cam follower member (25) and a gas spring (27). The balancer mechanism has a reaction force generation device (22) provided for generating, a balancing moment on the rotating shaft that cancels out a tilting moment that tilts the rotating shaft (4) around a bearing member (9) in a direction to separate it from the assistance torque generation device (21) and which is caused by the biasing force of the gas spring (27).