A tilt adjusting mechanism provided in a processing apparatus including a chuck table having a holding surface for holding a workpiece thereon and a processing unit having a processing surface for processing the workpiece held on the chuck table, for adjusting parallelism between the holding surface and the processing surface. The tilt adjusting mechanism includes at least three support posts supporting the chuck table, the support posts including at least one expansible and contractible support post that is provided with a piezoelectric actuator including layered piezoelectric elements, a direct current power supply electrically connected to the piezoelectric actuator, and a voltage controller for controlling a direct current voltage value of direct current electric power supplied to the piezoelectric actuator.