Abstract: The invention relates to an apparatus for determining the angle (?) between two planar workpiece surfaces (1, 2), comprising a laser transmitter (10) for a laser beam (9), a continuously rotating straightening rotor (3), which is arranged between the two workpiece surfaces (1, 2), is axially parallel to the vertex axis (5) of the angle (?) and has an emission direction for the laser beam (9) that is perpendicular to the rotor axis (4), and comprising a receiving device (11) having a receiver for the laser beam (9) reflected on the workpiece surfaces (1, 2) in the emission direction, and an evaluation circuit (18) connected to the receiving device (11). In order to provide advantageous constructive conditions, it is proposed that the laser transmitter (10) comprises a monitor diode (16) and that the monitor diode (16) forms the receiver for the reflected laser beam (9).