PURPOSE: To detect the relative angle with a target object with a simple structure and accurately without any contact.

CONSTITUTION: A light emitting part 1 for applying two fluxes of light L1 and L2 at a certain angle α to a measurement target surface so that two light points A and B are formed is constituted and the images of two light points A and B on the measurement target surface are picked up along with the background by a CCD image pick-up machine 4. Two light points A and B are detected out of the video signal of the CCD image pick-up machine 4 by a central control device 2. the amount of relative displacement of the light emitting part 1 for the measurement target surface is obtained by measuring the distance between the light points and an initial value which is input in advance, and then a displacement angel θ of the light emitting part 1 for the measurement target surface can be detected without any contact.