To provide an apparatus for simultaneously measuring a straightly acting motion and rotary angle error and a rotary angle error of a direct-acting table or another direct-acting unit without contact by utilizing a linearity of a laser beam.

A straightness measuring apparatus of the direct-acting unit comprises a horizontal parallel double mirror having two reflecting mirrors disposed in a horizontal plane in a state in which the mirrors are separated at a distance (√2)d from each other and reflecting surfaces are opposed, a vertical parallel double mirror having two reflecting mirrors disposed in a vertical plane in a state in which the mirrors are separated at a distance (√2)d from each other and reflecting surfaces are opposed, and a light source for generating the laser beam, a semitransparent mirror for splitting the beam, an optical path changing reflecting mirror for changing a direction of the beam and a position sensor for detecting a position of the reflected light from the horizontal parallel double mirror and the vertical parallel double mirror of a corner cube reflecting mirror at fixed positions out of the direct-acting unit.