A machining jig for rotatably supporting a workpiece comprising: a rotary drive part which rotates a seating part about a rotation axis, a clamp arm which has a rotary type clamp member which cooperates with the seating part to fasten a workpiece at a position separated from the seating part in a direction along the rotation axis of the seating part, a linear drive part which moves the clamp arm in a direction along a rotation axis, a position adjusting part which adjusts a position of the clamp arm in a plane intersecting the rotation axis, and a tilt adjusting part which adjusts a tilt of the clamp member so that a rotation axis of the clamp member becomes parallel with the rotation axis of the seating part.