Optical sensor and abnormality detection method for optical sensor

An abnormality detection method for an optical sensor that measures a distance to an object or a displacement of the object based on a reflected light from the object, the abnormality detection method comprising:

• irradiating light generated by a light source device to the object and acquiring a received light amount detected by a light receiving part as a reference detection amount, wherein a first optical fiber optically coupled to the light source device and a second optical fiber optically coupled to the light receiving part are fused at a branch part to join with one end of a third optical fiber which faces the object, wherein the reference detection amount is a received light amount detected by the light receiving part in a state of no reflection from the other end of the third optical fiber;
• irradiating light generated by the light source device to the object and acquiring a received light amount detected by the light receiving part as an evaluation object detection amount;
• and
• determining whether junction abnormality at a junction between the third optical fiber and the branch part based on whether an increment in the evaluation object detection amount with respect to the reference detection amount is within a predetermined range.