Noncontact automatic log sealer is a key instrument in process automation in the wood yard and timber mill.We introduce in this article a kind of opto-electronic apparatus for log diameter measurement named Nanlin -1 type, which has been successfully trial-produced by our research group. In this device, discontinuous parrallel scanning light beams are produced from a spiral surface with many small mirrors on it. These beams are forcused by a parabolic mirror and received by a photo-electric cell. As the calculation is made by a logic circuit based on the number of the scanning light beams shaded by the log, the diameter of the smaller end of the log is measured autometically. The apparatus measures accurately, the errors made in measurement being within 0.5 cm. It has a good repeatability and stable property. It can measure the smaller end of the log from 5 to 65 cm. in diameter, the resolution being 0.5 cm.