A mounting shaft device comprises a spline shaft including a supply passage for air pressure that extends through a shaft hole in a support, a conduit in communication with a valve and allowing the air pressure from the valve to be introduced to the supply passage, a component retainer disposed at an axial end of the spline shaft and configured to hold an electronic component by using the air pressure applied through the supply passage, a spline nut connected to the spline shaft through a spline mechanism, and a bearing supporting the spline shaft in the shaft hole in the support both in a linearly movable manner in an axial direction and in a rotatable manner about an axis thereof. The bearing, the conduit, and the spline nut are arranged on an axis of the spline shaft in this order from the component retainer.