The Photohead utilizes parallel light flux and detects the edge position of a strip (web) electrically. The two head blocks are independent: the lower block is composed of a projector consisting of a lamp and a lens and the upper block is composed of a detector incorporating a silicon photoelectric element. These blocks are of a cartridge type, and it is convenient to replace them when the lamp has blown or the photoelectric element has deteriorated. As the edge position of a strip (web), which is present in the parallel light flux between both blocks, changes, the parallel light flux is interrupted and the electrical signal of the photoelectric element changes continuously. Then, the signal is transmitted to the Universal Amplifier (or web guide amplifier) to activate the moving coil of the controller. When the periphery of the Photohead mounted is dusty, it is important to purge air in order to protect the lens surfaces of the projector and the detector. It is easy to purge air if an air blower or a pressure-reducing device for purging air is installed using a Photohead provided with the air purge mechanism.