Abstract: An inclination detector is constructed such that a displacement detection plate of generally fan-shape is pivotally supported at its fan-pivot, a plurality of slits are formed in the displacement detection plate concentrically of the fan-pivot at predetermined intervals, and a light emitting element and a light sensitive element constituting a photocoupler are disposed on opposite sides of the displacement detection plate with the slits interposed therebetween, whereby when a slit is caused to move due to a vibration or slant and traverse the optical path of the photocoupler, a displacement of the slit due to the vibration or slant is outputted from the light sensitive element.