By connecting a polyethylene fiber to a photo-multiplier, a beam loss monitor with a fast response has been fabricated. The beam loss is measured by Cherenkov light generated by a charged particle penetrating a fiber. Although this monitor has the merits that the structure of the monitor is simple and cheap, it has the demerits that the signal has an energy dependence, and deterioration caused by radiation occurs. A plastic scintillating fiber and a quartz fiber have also been tested. The signal of the former fiber is strong and does not depend on the energy of the circulating beam. However, the deterioration is also very severe. On the contrary, the latter fiber a long lifetime, though its signal is small. Some measurements are discussed here concerning various cases of extraction conditions.
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