To obtain an angle measuring instrument which is improved in measuring sensitivity to about double (reduced in minimum resolution to about half) and is hardly affected by a disturbance, such as the vibration, temperature change, etc.

In order to measure the inclined angle of an object to be measured from the image forming position of reflected light rays LR1 reflected by the first reflecting surface 21 of an object to be measured by emitting a parallel luminous flux LE toward the surface 21 and receiving the reflected light rays LR1, a second reflecting surface 22 which is arranged nearly perpendicularly to the first reflecting surface 21 is for measuring angle θ of a to-be- measured object provided. Then the inclined angle of the object is measured based on the shifting amount of the image forming position of the reflected light rays RL2 from the second reflecting surface 22 from that of the reflected light rays LR1 from the first reflecting surface 21.