This invention provides a sensor attachment device that can be easily attached and detached, a sensor and a light emitting device for a photo-electrical sensor. By downward displacement of an operation member after tip ends of a pair of locking hooks are locked to a sensor main body by inserting the sensor main body into an insertion opening with second regions of a pair of facing surfaces respectively facing toward external surfaces of central portions of the pair of locking hooks, the central portions of the pair of locking hooks can be pressed inwardly by first regions of the pair of facing surfaces, thereby fixing the sensor main body to a holding member. Accordingly, the attachment of the sensor main body is easy in comparison with the conventional configuration in which a nut is screwed onto the sensor main body, then rotated and fastened.