A balancer mechanism for a rotating shaft is capable of preventing deviation of the axis of the rotating shaft. The balancer mechanism (20) has an assistance torque generation means (21) for generating an assistance torque; the assistance torque generation means (21) includes a cam member (24) fixed to the rotating shaft (4), a cam follower member (25) and a gas spring (27); and a reaction force generation means (22) is provided for generating, by the biasing force of the gas spring (27), a balancing moment 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 means (21).
