An auxiliary cylinder apparatus installed in a work machine including a derricking member capable of making a derricking motion rotating in a derricking direction, comprising: an auxiliary cylinder attached to the derricking member so as to be capable of making a swing motion about an axis parallel to a rotation axis of the derricking motion to the derricking member, the auxiliary cylinder being capable of making expansion and contraction motions of expanding and contracting in expansion and contraction directions that intersect a swing axis of the swing motion; and a fixing device configured to be capable of fixing the auxiliary cylinder to the derricking member only when an expansion length which is a length of the auxiliary cylinder in the expansion and contraction directions is within a predetermined fixing allowing range and configured to release fixing of the auxiliary cylinder when the expansion length is deviated from the fixing allowing range; characterized in that the auxiliary cylinder apparatus further comprises an auxiliary-cylinder angle detector that detects an auxiliary-cylinder angle which is an angle of the expansion and contraction directions of the auxiliary cylinder to the horizontal direction; and an expansion controller that performs an expansion control which is a control of the expansion and contraction motions of the auxiliary cylinder, the expansion control including judging whether the auxiliary cylinder is in a predetermined vertical posture, based on the auxiliary-cylinder angle detected by the auxiliary-cylinder angle detector, allowing the auxiliary cylinder to make the expansion and contraction motions when the auxiliary cylinder is in the vertical posture, and prohibiting the auxiliary cylinder from the expansion and contraction motions that deviates the expansion length from the fixing allowing range, when the auxiliary cylinder is in a posture other than the vertical posture in a state where the auxiliary cylinder is fixed to the derricking member by the fixing device. an auxiliary cylinder attached to the derricking member so as to be capable of making a swing motion about an axis parallel to a rotation axis of the derricking motion to the derricking member, the auxiliary cylinder being capable of making expansion and contraction motions of expanding and contracting in expansion and contraction directions that intersect a swing axis of the swing motion; and a fixing device configured to be capable of fixing the auxiliary cylinder to the derricking member only when an expansion length which is a length of the auxiliary cylinder in the expansion and contraction directions is within a predetermined fixing allowing range and configured to release fixing of the auxiliary cylinder when the expansion length is deviated from the fixing allowing range; characterized in that the auxiliary cylinder apparatus further comprises an auxiliary-cylinder angle detector that detects an auxiliary-cylinder angle which is an angle of the expansion and contraction directions of the auxiliary cylinder to the horizontal direction; and an expansion controller that performs an expansion control which is a control of the expansion and contraction motions of the auxiliary cylinder, the expansion control including judging whether the auxiliary cylinder is in a predetermined vertical posture, based on the auxiliary-cylinder angle detected by the auxiliary-cylinder angle detector, allowing the auxiliary cylinder to make the expansion and contraction motions when the auxiliary cylinder is in the vertical posture, and prohibiting the auxiliary cylinder from the expansion and contraction motions that deviates the expansion length from the fixing allowing range, when the auxiliary cylinder is in a posture other than the vertical posture in a state where the auxiliary cylinder is fixed to the derricking member by the fixing device.