A linear motion shaft mechanism having a control shaft 35 linearly movable along an axial direction and a rotary-to-linear actuator 7 causing the control shaft 35 to move linearly. The actuator 7 includes a sun shaft 81 for transmitting linear motion to the control shaft 35, and a conversion mechanism 8 that converts rotational motion to linear motion and causes the sun shaft 81 to move linearly. The linear motion shaft mechanism includes a load applying portion. When a direction in which the sun shaft 81 is displaced from the conversion mechanism 8 toward the control shaft 35 is defined as a reference direction, the load applying portion causes a thrust load acting in the reference direction to act on the sun shaft 81 from the control shaft 35.